. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 4S FORMATION OF THE GERM LAYERS is much modified by the same cause, and to get a clear understanding of the formation of the triploblastic condition of the mammal it will be necessary to describe briefly its development in lower forms. In Amphioxus the diploblastic condition results from the flattening of the large-celled pole of the blastula (Fig. 23, A), and finally from the invagination of this portion of the vesicle within the other portion (Fig. 23, B). The original single-walled blastul
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 4S FORMATION OF THE GERM LAYERS is much modified by the same cause, and to get a clear understanding of the formation of the triploblastic condition of the mammal it will be necessary to describe briefly its development in lower forms. In Amphioxus the diploblastic condition results from the flattening of the large-celled pole of the blastula (Fig. 23, A), and finally from the invagination of this portion of the vesicle within the other portion (Fig. 23, B). The original single-walled blastula in this way becomes converted into a double-walled sac termed a gastrula, the outer layer of which is known as the ectoderm or epiblast and the inner layer as the endoderm or hypoblast. The cavity bounded by the endoderm is the primitive gut or archenteron, and the opening by which this communicates with the exterior is the blastopore. This last structure is at first a very wide opening, but as development proceeds it becomes smaller, and finally is a relatively small opening situated at the posterior extremity of what will be the dorsal surface of the embryo. As the oval embryo continues to elongate in its later development the third layer or mesoderm makes its appearance. It arises as a lateral fold imp) of the dorsal sur- face of the endoderm (en) on each side of the middle line as indicated in the transverse section shown in Fig. 24. This fold eventually be- comes completely constricted off from the endoderm and forms a hollow plate occupying the space between the ectoderm and endo- derm, the cavity which it contains being the body-cavity or coelom. In the amphibia, where the amount of yolk is very much greater than in Amphioxus, the gastrulation becomes considerably modified. On the line where the large- and small-celled portions of the blastula become continuous a crescentic groove appears and, deepening,. Fig. 24.—Transverse Section of A mphioxus Embryo with Five Mes
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