v.] THE BRAIN. 99 



A. epiblast. B. mesoblast. C. hypoblast consisting of a single 

 row of flattened cells, if. c. medullary canal. P. v. meso- 

 blastio somite. W. d. Wolffian duct. S. o. Somatopleure. 

 S.p. Splanchnopleure. p.p. pleuroperitoneal cavity, c.h. 

 notocbord. a. o. dorsal aorta, v. blood-vessels of the yolk- 

 sac, o.p. line of junction between opaque and pellucid 

 areas ; w. palisade-like yolk spheres which constitute the ger- 

 minal wall. 



Only one-half of the section is represented in the figure — if 

 completed it would be bilaterally symmetrical about the line of 

 the medullary canal. 



take place of course beneath the common epiblastic 

 investment, which is not involved in them. As a con- 

 sequence, though easily seen in the transparent fresh 



Head of a Chick at the End of the Second Day viewed 



fbom below as a transparent object. 



(Copied from Huxley). 



/. first cerebral vesicle, a. optic vesicle, d. infundibulum. 



The specimen shews the formation of the optic vesicles {a), 

 as outgrowths from the 1st cerebral vesicle or vesicle of the 3rd 

 ventricle, so that the optic vesicles and vesicle of the 3rd ven- 

 tricle at first freely commimicated with each other, and also the 

 growth of the lower wall of the vesicle of the 3rd ventricle into a 

 process which becomes the infundibulum {d). 



7—2 



