390 - Multicellular Animals, Especially Man 



grow until it envelops the embryo completely 

 (Fig. 21-1 IB). Then the amniotic folds meet 

 and fuse above the embryo, forming a new 

 cavity, which is called the amniotic cavity. 

 This amniotic cavity lies between the embryo 

 and the inner layer of the amniotic fold, and 

 is not to be confused with extraembryonic 

 coelom, which lies between the inner and 

 outer folds of the original outgrowth. After 

 the amniotic folds meet and fuse above the 

 embryo, the inner layer of the outgrowth is 

 called the amniotic membrane, and the outer 

 layer is called the chorion (Fig. 21-1 1C). 



The amniotic cavity is filled with an aque- 

 ous fluid secreted by the amniotic membrane. 

 Accordingly the embryo continues develop- 

 ment suspended in a watery medium, which 

 duplicates many of the conditions of a truly 

 aquatic environment. In eggs of the birds 

 and reptiles, the chorionic layer of the amni- 

 otic fold comes to lie in contact with the 

 inner surface of the egg shell (Fig. 21-12). 

 But in higher mammals, the chorion comes 

 into direct contact with the uterine wall, 

 which, after implantation, envelops the whole 

 embryo. 



The Allantois. The allantois, or second em- 

 bryonic membrane, arises as an outpocketing 

 from the enteron. As it grows, the allantois 

 continues to push out into the extraem- 



bryonic coelom, especially in a posterior 

 direction (Fig. 21-1 IB, C). Then the outer 

 layer of the allantois fuses intimately with 

 the chorion, forming a single highly vascular 

 membrane that lies immediately subjacent to 

 the egg shell (Fig. 21-12). 



The allantois is an embryonic organ that 

 fulfills the absorptive, respiratory, and ex- 

 cretory needs of the developing embryo. 

 Shortly after its formation, the allantois be- 

 comes permeated with blood capillaries that 

 are supplied and drained by major vessels 

 from the embryo proper. In the bird and 

 reptile, the allantoic capillaries serve not 

 only to take in oxygen and give off carbon 

 dioxide through the porous shells, but also 

 to absorb the material of the egg white. 

 Moreover, the allantois is a repository for 

 the nongaseous metabolic wastes that are 

 formed during embryonic development. At 

 the time of hatching, most of the allantois is 

 discarded, together with all of the amnion. 

 But the root of the allantois, at its junction 

 with the enteron, is retained in mammals, 

 being converted into part of the urinary 

 bladder of the adult. 



The Uterus and Placenta. The mono- 

 tremes, an almost extinct group of very 

 primitive mammals, have retained the ovi- 

 parous habit (Fig. 21-13), but a great ma- 



AMNIOTIC CAVITY 



AIR SPACE 



REMNANT OF 

 "EGG-WHITE" 

 YOLK SAC 



-SHELL 



Fig. 21-12. Embryonic membranes of a developing bird's egg. 



