542 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



distinctly thickened as compared with the outer or pigment Layer. 



and with a narrow optic stalk passing to the thalamencephalon 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 



FIG. 23/F. Transverse section passing through the the hinder limit of the 

 rudiments of the eye and otocyst. headfold of the SOmatO- 



plgure . The pharynx 

 * ^ 



paSSCS 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 



ihal 



a.c.u. 



- , anterior cardinal vein ; h.b, hind brain ; N, notochord ; 

 opt, optic cup; ot, otocyst; j>h, pharynx; v.c.I, iirst visceral 

 cleft ; r.ca, ventral carotid. 



Transverse section passing thn.ii-li the eye :IIM! just in IV. mt ..f tin- >t.u-y>t. 



"I inal vi-iii ; a., . :i>i t n 1 runt ; ./., ,i. ,[,,1 >al cai . .t i<l ;n l.-ry ; )/""" 



cranial II<T\-; /..'*, liiixl brain: ///. piinit.-n;. , notoohord^ //, I'liai ynx : /.///, 



thai, tlialaiiifiM-'-|ih;iluii : Oenl clrft. 





tionB -shows that il is still roniMvinl \\illi tin- 

 oiiicr skin hy a namm m-c.k. In the spongy connective tissue 

 which forms parkin^ l.rtwccn the \aridiis orLiaiis arc seen a 



