EMBRYO AT THE END OF THE FOURTH DAY SEEN AS 

 A TRANSPARENT OBJECT. 



The amnion has been completely removed, the cut end of the 

 somatic stalk is shewn at S.S. with the allantois (Al.} protruding 

 from it. 



C.ff. cerebral hemisphere. F.B. fore-brain or vesicle of the third 

 ventricle (thalamencephalon) with the pineal gland (Pn.) 

 projecting from its summit. M.B. mid-brain. Cb. cerebellum. 

 1 V. V. fourth ventricle. L. lens. ch.s. choroid slit. Owing to 

 the growth of the optic cup the two layers of which it is com- 

 posed cannot any longer be seen from the surface ; the pos- 

 terior surface of the choroid layer alone is visible. Cen. V. 

 auditory vesicle, s.m. superior maxillary process. IF, %F, etc. 

 first, second, third and fourth visceral folds. V. fifth nerve 

 sending one branch to the eye, the ophthalmic branch, and 



