102 TEXT-BOOK OF EMBRYOLOGY. 



converge to form other vessels which enter the embryo as thevitellineoromphalo- 

 mesenteric veins (Fig. 213). As the mesoderm extends farther and farther 

 around the yolk, the vessels extend likewise until the entire yolk is surrounded 

 by a dense plexus of blood vessels in the wall of the yolk sac. 



The Allantois. While the embryonic intestine is first assuming the form 

 of a tube, there grows out ventrally from near its caudal end, during the third 

 day of incubation, a diverticulum which is the beginning of the allantois (Fig. 

 99). This increases rapidly in size and pushes out into the extraembryonic 

 body cavity behind the yolk stalk. As it is a diverticulum from the intestine, 

 it consists primarily of entoderm. This pushes in front of it, however, the 

 splanchnic (visceral) mesoderm which becomes the outer layer of the membrane. 

 The connection between the intestine and the allantois is known as the urachus. 

 In the chick the allantois attains a comparatively large size, pushing out dorsally 



ex.b.c. -' 



al. p. / 



al. mes. ent. a.m. e.g. a.m. 



FIG. 99. Longitudinal section of caudal end of chick embryo (end of third 



day of incubation). Casser. 



al., Allantois; al. p., allantois prominence; a.m., anal membrane; am., amnion; am. c., amniotic 

 cavity; e.g., caudal gut; cce., ccelom; ect., ectoderm; ent., entoderm; ex. b. c., extraembryonic 

 body cavity; mes., mesoderm; pr. g., primitive gut; t., tail. 



between the amnion and the primitive chorion and ventrally between the latter 

 and the yolk sac (Fig. 97, b, c and d). The inner wall of the allantoic sac blends 

 with the amnion about the seventh day of incubation and with the yolk sac con- 

 siderably later, while the outer wall joins the primitive chorion to form the true 

 chorion, or as it is sometimes designated, the allanto-chorion (see p. 107). As 

 the allantois reaches the limit of the yolk, it leaves the latter, and pushing the 

 primitive chorion before it, continues around close under the shell (Fig. 97) 

 until it completely encloses the albumen at the small end of the egg. 

 The allantois of the chick performs three important functions : 



1. It serves as a receptacle for the excretions of the primitive kidneys. 



2. United with a part of the primitive chorion to form the albumen sac, its 

 vessels take up the albumen as nourishment for the embryo. Because of this 

 function and also because of the fact that little papillae sometimes appear on the 



