542 EMBEYOLOGY OF THE LOWEE VEETEBBATES oh. 



distinctly thickened as compared with the outer or pigment layer, 

 and with a narrow optic stalk passing to the thalameneephalon near 



its floor. In the mouth 

 of the optic cup is the lens 

 but this is seen better a 

 few sections farther on in 

 the series. 



Turning to the other 

 half of the section it is 

 seen that it is no longer 

 connected with the extra- 

 embryonic somatopleure : 

 in other words the series 

 of sections has now passed 

 the hinder limit of the 

 headfold of the somato- 

 pleure. The pharynx 

 passes out as a pocket 

 on each side towards the 

 ectoderm — the rudiments of the first pair of visceral clefts (v.c.I). 

 The neural tube has become greatly increased in size forming the 

 hind brain with its widely expanded cavity — the fourth ventricle. 

 On each side is a large thick-walled sac — the otocyst. Examination of 



a.c.v. 



Fig. 237p. — Transverse section passing through the 

 rudiments of the eye and otocyst. 



a.c.v, anterior cardinal vein ; h.o, hind brain ; N, notochord ; 

 opt, optic cup; ot, otocyst; p7i, pharynx; v.c.I, lirst visceral 

 cleft ; v.ca, ventral carotid. 



thai 



pm. 



a.cu. 



9<zng. ^ h 



ph. ir.cl i 

 ' a.; 



Fig. 237g. — Transverse section passing through the eye and just in front of the otocyst. 



a.c.v, anterior cardinal vein ; a.r, aortic root ; d.ca, dorsal carotid artery ; gang, ganglion of eighth 

 cranial nerve ; h.o, hind brain ; hy, pituitary body ; N, notochord ; ph, pharynx ; pin, pineal organ ; 

 thai, thalameneephalon ; v.c.I, first visceral cleft. 



neighbouring sections shows that it is still connected with the 

 outer skin by a narrow neck. In the spongy connective tissue 

 which forms packing between the various organs are seen a 



