v.] THE BRAIN. 99 



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

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

 blastic somite. W. d. Wolfl&an duct. S. o. Somatopleiure. 

 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 



Fig. 35. 



v 



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



FROM below as A TRANSPARENT OBJECT. 



(Copied from Huxley). 



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



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 communicated 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). 



72 



