208 



PRINCIPLES OF ANIMAL BIOLOGY 



the brain. These ridges approach one another and fuse along their 

 upper surfaces (Fig. 18 IB), cutting off a tube beneath the ectoderm. 

 In longitudinal vertical section at this time, the nervous system appears 

 as in Fig. 183. 



Fig. 180. — Mesoderm formation in frog. First three figures, median sections; last 

 figure, cross section. The hne-shaded cells on the outside, as they turn in, become the 

 mesoderm, a, edges of trough of archenteron. 



Beneath the nervous system a cylindrical rod of cells, the notochord, 

 is formed out of the middle portion of the inturned mesoderm. Around 



it later is formed the backbone. 

 The digestive tract has been pres- 

 ent, as the archenteron, ever 

 since gastrulation took place. At 

 first it is usually enlarged in front 

 and narrowed behind. These parts 

 correspond roughly, in the frog, to 

 the stomach and intestine. Poster- 

 iorly the intestine opens to the 

 outside through the anus, which in 

 some animals is the same opening 

 as the blastopore but in others a passage produced anew aftei- the 

 blastoi)or(i has closed. 



As indicated earlier (page 20()) and in Fig. 182, the mesoderm is early 



Fig. 181. — Neural folds of frog em- 

 bryo. A, folds still separate, brain above, 

 spinal cord below; B, folds fused, produc- 

 ing neuial tube beneath surface. 



