AYES. 



263 



3. Fate of the Germinal Layers. These give rise to the 

 same organs as in the frog (p. 226), but there are differences 

 which will be here briefly noted. (1) The ectoderm (epiblast) 

 does not become divided into superficial and nervous layers till 

 comparatively late, and the membranous labyrinth commences 

 as a pit open externally. (2) The endoderm (hypoblast) presents 

 no important differences, but the notochord disappears more corn- 



Fig. 80. DEVELOPMENT OF THE EYE (from ffaddon, A after Marshall). 

 A and B sections through head of third day chick. In A the optic 

 vesicle (o.c) from the fore-brain (/.&) is becoming cup-like, I is the 

 developing lens. In B the cup and lens are more distinct. Stalk of the 

 former not cut through. C. Later stage Inner wall of optic cup 

 (o.c) forming most of retina, r outer wall, p, forming pigment layer 

 of ditto; /, leus (quite separated off). These sections incidentally 

 illustrate other points. In all, owing to the cranial flexure, both fore- 

 and hind-brains have been cut through (f.b, h.b). In C, c./i, Lt, and 

 inf, indicate cerebral hemispheres, lamina terminalis, and infundi- 

 bulum. Near the last is the pituitary body, pit. In A and B the 

 pharynx (ph) is cut through, and the first two visceral clefts (v.c' and 

 v.c") are shown, a.a, aortic arches; a.c.v, anterior cardinal vein; 

 nch, notochord ; olj, olfactory pit. 



pletely in the adult. (3) The mesoderm (mesoblast) becomes 

 divided (cf.p. 227) into vertebral and lateral plates in the region of the 

 trunk, the former being again divided into mesodermic (mesoblastic) 

 somites (Figs. 78 and 79). The coelom (Fig. 81) is as before the 

 split between the somatic and splanchnic layers of the lateral 

 plates, the former uniting with the ectoderm to form the body-wall 

 (somatopleure), the latter with the endoderm to form the wall of 



