Note 133 — Chapter XIII

The wide occurrence of this process was first pointed out by Rabl. He holds, however, a peculiar modification of the gastræa theory, for which I must refer the reader to his paper (No. 284); according to this theory the mesoblast has sprung from a zone of cells of the blastosphere, at the junction between the cells which will be invaginated and the epiblast cells. In the bilateral blastosphere, from which he holds that all the higher forms (Bilateralia) have originated, these cells had a bilateral arrangement, and thus the bilateral origin of the mesoblast is explained. The origin of the mesoblast from the lips of the blastopore is explained by the position of its mother-cells in the blastosphere. It need scarcely be said that the views already put forward as to the probable mode of origin of the mesoblast, founded on the analogy of the Cœlenterata, are quite incompatible with Rabl’s theories.