ANIMAL BIOLOGY. 



[Part I. 



outer layer becomes somewhat thickened, forming the neural 

 plate. The edges of the thickened area rise up on either side so 

 as to form a median groove, the neural groove (32, iv. n. g.) ; 

 and then close over above the groove so as to convert it into a 



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FIG. 32. DIAGRAM OF EARLY STAGES OF DEVELOPMENT. 



i. and ii. Blastula stage, iii. Gaslrula stage, iv. Establishment of mesen- 

 teron ; notochord and medullary groove, v. The closure of the medullary 

 folds and cleavage of the mesoblast. 



ci^ e. Archenteron. 6. c. Body-cavity (coalom). Np. Blastopore. ch. Noto- 

 chord. ep. E pi blast, ty. Hypoblast. 1. p. Lateral plate, n. c. Neural 

 canal, n^g. Neural groove, m. p. Mesoblastic plate, tsf. Mesoblast. 

 mn. Mesehteron. s. c. Segmentation cavity, so. I. Somatic layer (somato- 

 pleure). ' sp. 1. Splanchnic layer (splanchnopleure). v. p. Vertebral plate. 



neural canal (32, v. n. c.). This is the first indication of. the 

 central nervous system. It lias in the mid-line of the dorsal 

 region, and is formed from* x %pT5lastT~ Immediately beneath it 

 there is formed a linear rod of cells derived from the hypoblast 

 of the mesenteron, and called the notocJwrd (ch.). 



Meanwhile a third germinal layer (Fig. 32, iv.) is being formed 

 between the epiblast and the hypoblast. It is the mesoblast. 

 The cells of which it is composed are probably derived from 

 the hypoblast; but in any case it soon forms a separate layer, 



