142 



THE FOURTH DAY. 



[CHAP. 



FIG. 46. 

 vir *^ 



*. s ;\ *rlr?* c * 



M.P 



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.H. cerebral hemisphere. F. B. fore brain or vesicle of the third ventricle 

 with the pineal gland (Pn.) projecting from its summit. M.B. mid brain. 

 Cb. cerebellum. IV. V. fourth ventricle. L. lens. ch.s. choroid slit. 

 Owing . to the growth of the optic cup the two layers of which it is 

 composed cannot any longer be seen from the surface, but the posterior 

 surface of the choroid layer alone is visible. Cen. V. auditory vesicle. 

 s.m. superior maxillary process. ijP. iF. etc. first, second, third and 

 fourth visceral folds. V. fifth nerve sending one branch to the eye, the 

 ophthalmic branch, and another to the first visceral arch. VII. seventh 

 nerve passing to the second visceral arch. G. Ph. glossopharyngeal nerve 

 passing towards the third visceral arch. Pg. pneumogastric nerve passing 

 towards the fourth visceral arch. iv. investing mass. No attempt has 

 been made in the figure to indicate the position of the dorsal wall of 

 the throat, which cannot be easily made out in the living embryo. 

 ch. notochord. The front end of this cannot be seen in the living embryo. 

 It does not end however as shewn in the figure, but takes a sudden bend 

 downwards and then terminates in a point. Ht. heart seen through the 

 walls of the chest. M. P. muscle-plates. W. wing. H. L. hind limb. 

 Beneath the hind limb is seen the curved tail. 



