Note 1 — INTRODUCTION.

In the embryos of reptiles, birds and mammals folds of the somatopleure arise externally to the constricting furrows by means of which the embryo is gradually separated from the yolk-sac, with the resulting formation of the intestinal and abdominal walls. These folds, situated at the head, tail and on the sides, grow upwards and finally meet and unite to form a membranous sac called the amnion. Hence these higher vertebrates (reptiles, birds and mammals) are called Amniota, in contradistinction to fishes and amphibia who have no amnion and are hence known as Anamnia.