IV.] 



THE TAIL-FOLD. 



75 



cavity, c. h. Dotochord. a. o. dorsal aorta, v. blood-vessels of the 

 yolk-sac, o. p. line of junction between opaque and pellucid areas ; 

 at this point the hypoblast cells are seen to pass without any strong 

 line of demarcation into the white-yolk spheres, w. white-yolk spheres, 

 some of which near the edge of the pellucid area contain a body very 

 like a nucleus. 



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. 



*. FIG. 21. 



op.v? 



EMBRYO OP THE CHICK AT 36 HOURS VIEWED FROM ABOVE AS AN OPAQUE 



OBJECT. (Chromic acid preparation.) 

 /. b. front brain. ra. b. mid brain. h. b. hind brain, op. v. optic vesicle. 



au. p. auditory vesicle, o. f. omphalo-mesaraic vein. p. v. protovertebra. 



m. f. line of junction of the medullary folds above the medullary canal. 



s. r. sinus rhomboidalis. t. tail-fold, p. r. remains of primitive groove. 



a. p. area pellucida. 



embryo being simply a part of a flat blastoderm, nearly 

 completely level from the front protovertebrse to the hind 

 edge of the pellucid area. At this epoch, however, a tail-fold 



