118 TEXT-BOOK OF EMBRYOLOGY. 



The Allantois. 



The human allantois, while analogous to the allantois of Birds and Reptiles, 

 shows certain marked peculiarities in its development, in its relation to sur- 

 rounding structures and in its functions. 



Its development is peculiar in that it does not push out, as, for example, in 

 the chick, as an evagination from the primitive gut into the extraembryonic 

 body cavity, for at the very early stage at which the human allantois first ap- 

 pears, the primitive gut is not as yet constricted off from the yolk sac and there 

 is no extraembryonic body cavity into which the allantois can extend. It will be 

 remembered that in the formation of the germ layers and in the development of 

 the amnion the human embryo shows a marked tendency, as compared with 

 lower forms, toward a shortening of the developmental process. This ab- 

 breviation and consequent very early formation applies also to the allantois. As 

 the embryonic body assumes definite shape and the amnion is formed, there is 

 not the complete separation of amnion from the chorion seen, for example, in the 

 chick, the embryo remaining connected posteriorly with the chorion by means of 

 a short thick cord of mesodermic tissue. This is known as the belly stalk. Into 

 this solid cord of mesodermic tissue which connects the embryo with the 

 chorion, entodermic cells extend. These are derived from the embryonic en- 

 toderm before the constriction which differentiates the primitive gut from the 

 yolk sac has made its appearance (Fig. 85). According to some there is a true 

 evagination from the entodermic sac quite analogous to the evagination in the 

 chick, resulting in a long slender tube lined by entoderm and extending from 

 the embryo to the chorion. Others describe the entodermic outgrowth as a 

 solid cord of cells. The mesodermic layer of the allantois is furnished by the 

 mesoderm of the belly stalk. It is to be noted in this connection that the 

 mesoderm of the belly stalk is embryonic mesoderm and that in Birds, for 

 example, this portion of the mesoderm splits into two layers, somatic and 

 splanchnic, with the extraembryonic body cavity between them. Into this 

 extraembryonic body cavity the allantois extends. In man no such splitting 

 occurs, so that there is no extraembryonic body cavity into which the allantois 

 can extend. Instead, it grows out into the belly stalk. 



The functions of the human allantois are somewhat different from those of 

 the allantois of the chick. In the latter it is a direct respiratory organ in that it 

 brings the embryo into relation with the outside air. In man the allantois, 

 accompanied by the allantoic (umbilical) blood vessels, comes into relation 

 with the placenta. As the placenta serves as the medium of exchange between 

 fcetal and maternal circulations, it acts as a modified organ of respiration. In 

 the chick the allantoic cavity also serves for the reception of the excretions from 

 the embryo, the allantoic fluid containing nitrogenous excretives. In man all 



