THE GERM-VESICLE BECOMES THE YELK-SAC. 



337 



The reversed convex keel represents the middle line of the 

 back; the little chamber under the fore-deck represents 

 the head intestinal cavity, and that under the after-deck 

 the pelvic intestinal cavity. (Cf. Fig. 94, p, 312.) 



Fig. 102. — Longitudinal section through the front half of a chick 

 (at the end of the first day of incubation), seen from the left side : k, head- 

 plates ; ch, notochord ; above the latter, the blind front end of the 

 medullary tube (mr) ; below it the head intestinal cavity, the blind front 

 end of the intestinal tube ; d, intestinal-glandular layer ; df, intestinal- 

 fibrous layer ; h, horn-plate ; hh, heart-cavity ; hk, heart-cap ; ks, head- 

 sheath ; kk, head-cap. (After Eemak.) 



With its two free ends the embryo now presses 

 somewhat into the external surface of the germ-vesicle, 

 and at the same time lifts the middle portion away from the 

 germ-vesicle. The consequence is that the germ-vesicle 

 soon appears to be merely a pouch-shaped appendage pro- 

 truding from the middle of the body. This appendage, which 

 continues to decrease in size, is afterwards called the yelk- 

 sac, or navel- vesicle. (Cf. Fig. 94, 4 , 5 , ds; Fig. 100, and Plate 

 V. Fig. 14.) The cavity of this yelk-sac, or cavity of the 

 germ-vesicle, communicates with the growing intestinal 

 cavity through a wide connecting aperture, which after- 



