IV.] 



THE TAIL-FOLD. 



75 



cavity, c. h. notochord. 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. 

 Oidy one half of the section is represented in the figure — if completed it 

 would be bilaterally symmetrical about the line of the medullary caual. 



Fig. 21. 



ElIBRYO OF THE CHICK AT 36 HOURS VIEWED FROM ABOVE AS AN OPAQUE 



Object. (Chromic acid preparation.) 

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

 au. p. auditory vesicle. 0. /. 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 prot overt ebrse to the hind 

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



