Allantois. 133 



sac, entirely covering the embryo above. The embryo sinks 

 further into the yolk, which is being continually absorbed, and 

 is, as stated, covered above by the amnion. Of this double- 

 layered sac the part nearest to the embryo, which it closely 

 invests, is known as the true amnion ; the outer layer becomes 

 pressed against the egg-shell, and this portion is termed ^q false 

 amnion. The space between these two layers is clearly coelom. 



AN ™ 



Fig. 70. — Embryo Chick at ninth day. (From Marshall.) 



AN, true amnion ; ta, allantois'; hm, hyomandibular cleft ; YS, yolk-sac ; wa, white 

 of the egg ; sv, air chamber. 



Another embryonic organ, found in the frog as well as in the 

 higher vertebrates, is the allantois (see Fig. 70, ta). The 

 allantois is an outgrowth of the gut ; it is therefore lined with 

 hypoblast, and covered externally by splanchnopleuric meso- 

 blast. It grows into a large thin-walled sac, which comes into 

 close contact with the shell, and serves as a respiratory organ 

 for the growing embryo, absorbing, as it does, into its numerous 

 blood capillaries the outside air. The allantois totally dis- 

 appears in the adult fowl ; in the frog it is permanently retained 

 as the bladder. 



