98 THE TOPOGENESIS OF THE EMBRYO 



animal field expands strongly, and finally covers the whole 

 surface of the germ. It also becomes thinner, and the multi- 

 layered arrangement of the cells is replaced by a single-layered 

 one. At the end of gastrulation, therefore, the embryo consists 

 of (1) an external ectoderm, or outer germ-layer, originating 

 from the animal field, (2) an internal endoderm, or inner germ- 

 layer, forming the floor and the sides of the archenteron, and 

 arising from the original vegetative field, and (3) the roof of 

 the archenteron, formed by the invaginated marginal zone 

 material, and constituting the middle germ-layer, or chorda- 

 mesoderm. The archenteron roof, consisting of the prospective 

 notochord and mesoderm sheets, extends ventrally between 

 ectoderm and endoderm (Fig. 34f). The main axes of the embryo 

 have now become visible: The blind end of the archenteron 

 marks the anterior end of the embryo; here the mouth will 

 break through. The blastopore lies at the posterior end, and 

 will become the anus. The back of the embryo will be formed 

 at the dorsal side of the germ, and its belly at the opposite side. 

 At the conclusion of gastrulation, topogenesis has not yet 

 come to an end. The next phase, known as neurulation, follows 

 immediately. During this period, new movements result in the 

 formation of the first organ systems of the embryo from the 

 material brought into place by gastrulation. The dorsal ecto- 

 derm thickens and becomes the neural plate (Fig. 34 g). The 

 edges of this plate fold upwards (neural folds), and finally 

 fuse, forming the neural tube. This tube is the primordium of the 

 embryonic central nervous system. The rest of the ectoderm 

 is the prospective epidermis. The mid-dorsal cells of the 

 archenteron roof, under the middle of the neural plate, become 

 free from the rest of the roof, and form a longitudinal cylin- 

 drical rod, the notochord. The adjoining strips, to the right 

 and left of the notochord, first thicken, and then divide into 

 a number of segments, the so-called somites, lying in a longi- 

 tudinal row. These will give rise to the musculature of the 

 trunk, etc. The remaining, lateral and ventral parts of the 

 mesodermal sheets supply the lateral mesoderm in which the 

 body cavity of the embryo will develop. Initially, the endoderm 

 was trough-shaped, but now its edges approach each other 



