Chapter IX · (continued 2)

The nerves then give us indications of seven cranial segments, or, if the nerves to the eye-muscles be included, of at the least eight segments, but to these must be added a number of segments now lost, but which once existed behind the last of those at present remaining.

The branchial clefts have been regarded as guides to segmentation by Gegenbaur, Huxley, Semper, etc., and this view cannot I think be controverted. In Scyllium there are six clefts which give indications of seven segments, viz., the segments of the mandibular arch, hyoid arch, and of the five branchial arches. If, following the views of Dr Dohrn[302], we regard the mouth as representing a cleft, we shall have seven clefts and eight segments; and it is possible, as pointed out in Dr Dohrn's very suggestive pamphlet, that remnants of a still greater number of præoral clefts may still be in existence. Whatever may be the value of these speculations, such forms as Hexanchus and Heptanchus and Amphioxus make it all but certain that the ancestors of Vertebrates had a number of clefts behind those now developed.

The last group of organs to be dealt with for our present question is that of the Head-Cavities.

The walls of the spaces formed by the cephalic prolongations of the body-cavity develop into muscles and resemble the muscle-plates of the trunk, and with these they must be identified, as has been already stated. As equivalent to the muscle-plates, they clearly are capable of serving as very valuable guides for determining the segmentation of the head. There are then a pair of these in front of the mandibular arch, a pair in the mandibular arch, and a pair in each succeeding arch. In all there are eight pairs of these cavities representing eight segments, the first of them præoral. As was mentioned above, each of the sections of the head-cavity (except perhaps the first) stands in a definite relation to the nerve and artery of the arch in which it is situated.

The comparative results of these three independent methods of determining the segmentation of the head are in the subjoined table represented in a form in which they can be compared:—

Table of the Cephalic Segments as determined by the Nerves, Visceral Arches, and Head-Cavities.

Segments Nerves Visceral Arches Head-Cavities or Cranial Muscle-Plates

Præoral 1 3rd and 4th and ? 6th nerves (perhaps representing more than one segment) (?) 1st head cavity (in my figures 1pp.)

Postoral 2 5th nerve Mandibular 2nd head-cavity (in my figures 2pp.)

——3 7th nerveHyoid 3rd head-cavity

——4Glossopharyngeal nerve 1st branchial arch 4th head-cavity

——5 1st branch of vagus 2nd branchial arch 5th head-cavity

——6 2nd branch of vagus 3rd branchial arch 6th head-cavity

——7 3rd branch of vagus 4th branchial arch 7th head-cavity

——8 4th branch of vagus 5th branchial arch 8th head-cavity

In the above table the first column denotes the segments of the head as indicated by a comparison of the three sets of organs employed. The second column denotes the segments as obtained by an examination of the nerves; the third column is for the visceral arches (which lead to the same results as, but are more convenient for our table than, the visceral clefts), and the fourth column is for the head-cavities. It may be noticed that from the second segment backwards the three sets of organs lead to the same results. The head-cavities indicate one segment in front of the mouth, and now that the ophthalmic branch of the fifth has been dethroned from its position as a separate nerve, the eye-nerves, or one of them, may probably be regarded as belonging to this segment. If the suggestion made above (p. 431), that the walls of the first cavity become the eye-muscles, be correct, the eye-nerves would perhaps after all be the most suitable nerves to regard as belonging to the segment of the first head-cavity.

EXPLANATION OF PLATES 15, 16, 17.

Plate 15. (The Head during stages G—K.)

Complete List of Reference Letters.

1aa, 2aa, etc. 1st, 2d, etc. aortic arch. acv. Anterior cardinal vein. al. Alimentary canal. ao. Aorta. au. Thickening of epiblast to form the auditory pit. aun. Auditory nerve. aup. Auditory pit. auv. Auditory vesicle. b. Wall of brain. bb. Base of brain. cb. Cerebellum. cer. Cerebrum. Ch. Choroid slit. ch. Notochord. com. Commissure connecting roots of vagus nerve. 1, 2, 3 etc. eg. External gills. ep. External epiblast. fb. Fore-brain. gl. Glossopharyngeal nerve. hb. Hind-brain. ht. Heart. hy. Hyaloid membrane. In. Infundibulum. l. Lens. M. Mouth involution. m. Mesoblast at the base of the brain. mb. Mid-brain. mn. v. Mandibular branch of fifth. ol. Olfactory pit. op. Eye. opn. Optic nerve. opv. Optic vesicle. opth V. Ophthalmic branch of fifth. p. Posterior root of spinal nerve. pn. Pineal gland. 1, 2 etc. pp. First, second, etc. section of body-cavity in the head. pt. Pituitary body. so. Somatopleure. sp. Splanchnopleure. spc. Spinal cord. Th. Thyroid body. v. Blood-vessel. iv. v. Fourth ventricle. v. Fifth nerve. Vc. Visceral cleft. Vg. Vagus. vii. Seventh or facial nerve.

Fig. 1. Head of a Pristiurus embryo of stage K viewed as a transparent object.

The points which deserve special attention are: (1) The sections of the body-cavity in the head (pp): the first or premandibular section being situated close to the eye, the second in the mandibular arch. Above this one the fifth nerve bifurcates. The third at the summit of the hyoid arch.

The cranial nerves and the general appearance of the brain are well shewn in the figure.

The notochord cannot be traced in the living embryo so far forward as it is represented. It has been inserted according to the position which it is seen to occupy in sections.

Fig. 2. Head of an embryo of Scyllium canicula somewhat later than stage K, viewed as a transparent object.

The figure shews the condition of the brain; the branches of the fifth and seventh nerves (v. vii.); the rudiments of the semicircular canals; and the commencing appearance of the external gills as buds on both walls of 2nd, 3rd, and 4th clefts. The external gills have not appeared on the first cleft or spiracle.

Fig. 3. Section through the head of a Pristiurus embryo during stage G. It shews (1) the fifth nerve (v.) arising as an outgrowth from the dorsal summit of the brain. (2) The optic vesicles not yet constricted off from the fore-brain.

Figs. 4a and 4b. Two sections through the head of a Pristiurus embryo of stage I. They shew (1) the appearance of the seventh nerve. (2) The portion of the body-cavity belonging to the first and second visceral arches. (3) The commencing thickening of epiblast to form the auditory involution.

In 4b, the posterior of the two sections, no trace of an auditory nerve is to be seen.

Figs. 5a and 5b. Two sections through the head of a Torpedo embryo with 3 visceral clefts. Zeiss A, ocul. 1.

5a shews the formation of the thin roof of the fourth ventricle by a divarication of the two lateral halves of the brain.

Both sections shew the commencing formation of the thyroid body (th) at the base of the mandibular arch.

They also illustrate the formation of the visceral clefts by an outgrowth from the alimentary tract without any corresponding ingrowth of the external epiblast.

Fig. 6. Section through the hind-brain of a somewhat older Torpedo embryo. Zeiss A, ocul. 1.

The section shews (1) the attachment of a branch of the vagus to the walls of the hind-brain. (2) The peculiar form of the hind-brain.

Fig. 7. Transverse section through the head of a Pristiurus embryo belonging to a stage intermediate between I and K, passing through both the fore-brain and the hind-brain. Zeiss A, ocul. 1.

The section illustrates (1) the formation of the pituitary body (pt) from the mouth involution (m), and proves that, although the wall of the throat (al) is in contact with the mouth involution, there is by this stage no communication between the two. (2) The eye. (3) The sections of the body-cavity in the head (1pp, 2pp). (4) The fifth nerve (v.) and the seventh nerve (vii).

Fig. 8. Transverse section through the brain of a rather older embryo than fig. 7. It shews the ventral junction of the anterior sections of the body-cavity in the head (1pp).

Figs. 9a and 9b. Two longitudinal sections through the brain of a Pristiurus embryo belonging to a stage intermediate between I and K. Zeiss A, ocul. 1.

9a is taken through the median line, but is reconstructed from two sections. It shews (1) The divisions of the brain—The cerebrum and thalamencephalon in the fore-brain; the mid-brain; the commencing cerebellum in the hind-brain. (2) The relation of the mouth involution to the infundibulum. (3) The termination of the notochord.

9b is a section to one side of the same brain. It shews (1) The divisions of the brain. (2) The point of outgrowth of the optic nerves (opn). (3) The sections of the body-cavity in the head and the bifurcation of the optic nerve over the second of these.

Fig. 10. Longitudinal section through the head of a Pristiurus embryo somewhat younger than fig. 9. Zeiss a, ocul. 4. It shews the relation of the nerves and the junction of the fifth, seventh, and auditory nerves with the brain.

Fig. 11. Longitudinal section through the fore-brain of a Pristiurus embryo of stage K, slightly to one side of the middle line. It shews the deep constriction separating the thalamencephalon from the cerebral hemispheres.

Fig. 12. Longitudinal section through the base of the brain of an embryo of a stage intermediate between I and K.

It shews (1) the condition of the end of the notochord; (2) the relation of the mouth involution to the infundibulum.

Fig. 13a. Longitudinal and horizontal section through part of the head of a Pristiurus embryo rather older than K. Zeiss A, ocul. 1.

The figure contains the eye cut through in the plane of the choroid slit. Thus the optic nerve (opn) and choroid slit (ch) are both exhibited. Through the latter is seen passing mesoblast accompanied by a blood-vessel (v). Op represents part of the optic vesicle to one side of the choroid slit.

No mesoblast can be seen passing round the outside of the optic cup; and the only mesoblast which enters the optic cup passes through the choroid slit.

Fig. 13b. Transverse section through the last arch but one of the same embryo as 13a. Zeiss A, ocul. 1.

The figure shews (1) The mode of formation of a visceral cleft without any involution of the external skin. (2) The head-cavity in the arch and its situation in relation to the aortic arch.

Fig. 14. Surface view of the nasal pit of an embryo of same age as fig. 13, considerably magnified. The specimen was prepared by removing the nasal pit, flattening it out and mounting in glycerine after treatment with chromic acid. It shews the primitive arrangement of the Schneiderian folds. One side has been injured.

Figs. 15a and 15b. Two longitudinal and vertical sections through the head of a Pristiurus embryo belonging to stage K. Zeiss a, ocul. 3.

15a is the most superficial section of the two. It shews the constitution of the seventh and fifth nerves, and of the intestinal branch of the vagus. The anterior branch of the seventh nerve deserves a special notice.

15b mainly illustrates the dorsal commissure of the vagus nerve (com) continuous with the dorsal commissures of the posterior root of the spinal nerves.

Fig. 16. Two longitudinal and vertical sections of the head of a Pristiurus embryo belonging to the end of stage K. Zeiss a, ocul. 1.

16a passes through the median line of the brain and shews the infundibulum, notochord and pituitary body, etc.

The pituitary body still opens into the mouth, though the septum between the mouth and the throat is broken through.

16b is a more superficial section shewing the head-cavities pp 1, 2, 3, and the lower vagus commissure.

Plate 16.

Complete List of Reference Letters.

auv. Auditory vesicle. cb. Cerebellum. cer. Cerebral hemispheres. ch. Notochord. cin. Internal carotid. ft. Fasciculi teretes. in. Infundibulum. lv. Lateral ventricle. mb. Mid-brain, or optic lobes. md. Medulla oblongata. mn. Mandible. ol. Olfactory pit. oll. Olfactory lobe. op. Eye. opn. Optic nerve. opth. Optic thalamus. pc. Posterior commissure. pcl. Posterior clinoid. pn. Pineal gland. pt. Pituitary body. rt. Restiform tracts. tv. Tela vasculosa of the roof of the fourth ventricle. iv. v. Fourth ventricle. vii. Seventh nerve. x. Rudiment of septum which will grow backwards and divide the unpaired cerebral rudiment into the two hemispheres.

Figs. 1a, 1b, 1c. Longitudinal sections of the brain of a Scyllium embryo belonging to stage L. Zeiss a, ocul. 1.

1a is taken slightly to one side of the middle line, and shews the general features of the brain, and more especially the infundibulum (in) and pituitary body (pt).

1b is through the median line of the pineal gland.

1c is through the median line of the base of the brain, and shews the notochord (ch) and pituitary body (pt); the latter still communicating with the mouth. It also shews the wide opening of the infundibulum in the middle line into the base of the brain.

Fig. 2. Section through the unpaired cerebral rudiment during stage O, to shew the origin of the olfactory lobe and the olfactory nerve. The latter is seen to divide into numerous branches, one of which passes into each Schneiderian fold. At its origin are numerous ganglion cells represented by dots. Zeiss a, ocul. 2.

Fig. 3. Horizontal section through the three lobes of the brain during stage O. Zeiss a, ocul. 2.

The figure shews (1) the very slight indications which have appeared by this stage of an ingrowth to divide the cerebral rudiment into two lobes (x): (2) the optic thalami united by a posterior commissure, and on one side joining the base of the mid-brain, and behind them the pineal gland: (3) the thin posterior wall of the cerebral rudiment with folds projecting into the cerebral cavity.

Figs. 4a, 4b, 4c. Views from the side, from above, and from below, of a brain of Scyllium canicula during stage P. In the view from the side the eye (op) has not been removed.

The bilobed appearance both of the mid-brain and cerebellum should be noticed.

Fig. 5. Longitudinal section of a brain of Scyllium canicula during stage P. Zeiss a, ocul. 2.

There should be noticed (1) the increase in the flexure of the brain accompanying a rectification of the cranial axis; (2) the elongated pineal gland, and (3) the structure of the optic thalamus.

Figs. 6a, 6b, 6c. Views from the side, from above, and from below, of a brain of Scyllium stellare during a slightly later stage than Q.

Figs. 7a and 7b. Two longitudinal sections through the brain of a Scyllium embryo during stage Q. Zeiss a, ocul. 2.

7a cuts the hind part of the brain nearly through the middle line; while 7b cuts the cerebral hemispheres and pineal gland through the middle.

In 7a the infundibulum (1), cerebellum (2), the passage of the restiform tracts (rt) into the cerebellum (3), and the rudiments of the tela vasculosa (4) are shewn. In 7b the septum between the two lobes of the cerebral hemispheres (1), the pineal gland (2), and the relations of the optic thalami (3) are shewn.

Figs. 8a, 8b, 8c, 8d. Four transverse sections of the brain of an embryo slightly older than Q. Zeiss a, ocul. 1.

8a passes through the cerebral hemispheres at their junction with the olfactory lobes. On the right side is seen the olfactory nerve coming off from the olfactory lobe. At the dorsal side of the hemispheres is seen the pineal gland (pn).

8b passes through the mid-brain now slightly bilobed, and the opening into the infundibulum (in). At the base of the section are seen the optic nerves and their chiasma.

8c passes through the opening from the ventricle of the mid-brain into that of the cerebellum. Below the optic lobes is seen the infundibulum with the rudiments of the sacci vasculosi.

8d passes through the front end of the medulla, and shews the roots of the seventh pair of nerves, and the overlapping of the medulla by the cerebellum.

Plate 17.

Complete List of Reference Letters.

vii. a. Anterior branch of seventh nerve. ar. Anterior root of spinal nerve. auv. Auditory vesicle. cer. Cerebrum. ch. Notochord. ch. Epithelial layer of choroid membrane. gl. Glossopharyngeal nerve. vii.hy. Hyoid branch of seventh nerve. hym. Hyaloid membrane. ll. Lateral line. v. mn. Ramus mandibularis of fifth nerve. vii. mn. Mandibular (spiracular) branch of seventh nerve. v. mx. Ramus maxillæ superioris of fifth nerve. nl. Nervus lateralis. ol. Olfactory pit. op. Eye. v. opth. Ramus ophthalmicus of fifth nerve. pch. Parachordal cartilage. pfal. Processus falciformis. pp. Head cavity. pr. Posterior root of spinal nerve. rt. Retina. sp. Spiracle. v. Fifth nerve. vii. Seventh nerve. vc. Visceral cleft. vg. Vagus nerve. vg.br. Branchial branch of vagus. vgcom. Commissure uniting the roots of the vagus, and continuous with commissure uniting the posterior roots of the spinal nerves. vgr. Roots of vagus nerves in the brain. vgin. Intestinal branch of vagus. vh. Vitreous humour.

Fig. 1. Diagram of cranial nerves at stage L.

A description of the part of this referring to the vagus and glossopharyngeal nerves is given at p. 426. It should be noticed that there are only five strands indicated as springing from the spinal cord to form the vagus and glossopharyngeal nerves. It is however probable that there are even from the first a greater number of strands than this.

Fig. 2. Section through the hinder part of the medulla oblongata, stage between K and L. Zeiss A, ocul. 2.

It shews (1) the vagus commissure with branches on one side from the medulla: (2) the intestinal branch of the vagus giving off a nerve to the lateral line.

Fig. 3. Longitudinal and vertical section through the head of a Scyllium embryo of stage L. Zeiss a, ocul. 2.

It shews the course of the anterior branch of the seventh nerve (vii.); especially with relation to the ophthalmic branch of the fifth nerve (v. oth).

Figs. 4a and 4b. Two horizontal and longitudinal sections through the head of a Scyllium embryo belonging to stage O. Zeiss a, ocul. 2.

4a is the most dorsal of the two sections, and shews the course of the anterior branch of the seventh nerve above the eye.

4b is a slightly more ventral section, and shews the course of the fifth nerve.

Fig. 5. Longitudinal and horizontal section through the hind-brain at stage O, shewing the roots of the vagus and glossopharyngeal nerves in the brain. Zeiss B, ocul. 2.

There appears to be one root in the brain for the glossopharyngeal, and at least six for the vagus. The fibres from the roots divide in many cases into two bundles before leaving the brain. Swellings of the brain towards the interior of the fourth ventricle are in connection with the first five roots of the vagus, and the glossopharyngeal root; and a swelling is also intercalated between the first vagus root and the glossopharyngeal root.

Fig. 6. Horizontal section through a part of the choroid slit at stage P. Zeiss B, ocul. 2.

The figure shews (1) the rudimentary processus falciformis (pfal) giving origin to the vitreous humour; and (2) the hyaloid membrane (hym) which is seen to adhere to the retina, and not to the vitreous humour or processus falciformis.