94 



THE TOPOGENESIS OF THE EMBRYO 



Fig. 32. The normal development of Psammechinus micro tuber- 



culatus. (a) early blastula; (b-c) formation of the primary mesen- 

 chyme (p.m.); id) beginning invagination of the archenteron; (e) 

 gastrula, from the left; formation of the secondary mesenchyme 

 (s.m.); (/) older gastrula, dorsal view; formation of the skeleton; 

 (g) the archenteron bends towards the ventral side; (h) formation 

 of the mouth (m). After Schmidt. 



invagination, and later become disengaged from the tip of the 

 archenteron. Originally, the archenteron is straight, but later 

 its terminal part bends, and comes into contact with the ecto- 

 derm (Fig. 32 g, h). At this point, an invagination, called the 

 stomodaeum, is formed by the ectoderm. This breaks through 

 into the archenteron, thereby forming the definitive mouth. 

 The archenteron becomes the gut, the blastopore becomes the 

 anus. The symmetry of the egg which, though preformed, was 

 not yet visible, manifests itself in these processes. The embryo 

 is now bilaterally symmetrical. The mouth has formed at its 

 ventral side ; the opposite side is dorsal. The cells of the primary 



