44 FORMATION OF THE ALLANTOIS. 



although they minister to its protection and nutrition, take no part in the formation 

 of any of its organs. But the case is different with the structure next to be 

 described, viz., the allantois, a part of which does in fact eventually become 

 converted into portions of the urinary and generative systems, although the greater 

 part is also extra-embryonic, its function being to minister, through its accompanying 

 blood-vessels, to the nutritive and respiratory exchanges of the foetus. 



The time of development of the allantois seems to vary much in mammals, and 

 there is reason to believe that it is found in the human embryo at a very early 

 period indeed the earliest human embryos that have hitherto been described 

 already possess an allantois. In most animals in which its development has been 

 studied the allantois has been found to begin as a hollow prolongation of the posterior 

 end of the primitive alimentary canal (fig. 46). It soon, however, becomes relatively 



Fig. 46. LONGITUDINAL SECTION THROUGH THE POSTERIOR END OF AN EMBRYO RABBIT, SHOWING THE 



OUTGROWTH OP THE ALLANTOIS. (Kolliker.) 



h, epiblast of trunk ; dd, hypoblast ; m, medullary or neural canal ; c7t, notochord ; hd, 

 commencing hind-gut, which is becoming formed by a folding-over of the tail end of the embryonic 

 blastoderm, v ; ed, blind end of hind gut ; al, allantois growing out from hind gut into aw, mesoblastic 

 thickening ; e, epithelium of yolk sac ; df, splanchnopleure ; hp, somato-pleure passing superiorly into 

 am, tail fold of amnion ; s, is placed within the cavity of the amnion, and denotes the tail end of the 

 embryo. 



shifted in position so as to come off from the ventral wall of the hind-gut, growing 

 into the posterior extension of the mesoblastic cleft and eventually into the space 

 between the false and true amnion, and carrying along with it its mesoblastic covering 

 (figs. 36, 45). It is therefore composed eventually of two parts, viz., (1) a hypo- 

 blastic sac which communicates, at first widely but afterwards by a narrowed orifice, 

 with the hind-gut, and (2) an investment of mesoblast. This last is usually greatly 

 thickened and very vascular, and is directly supplied with blood by two arteries 

 (allantoic or umbilical arteries), which appear at first as a direct continuation of 

 the primitive aortae. As the allantoic vesicle expands into the cavity of the false 

 amnion, it carries the vascular mesoblast along with it, so that this mesoblast is thus 

 brought to the inner surface of the chorion, over which its blood-vessels then spread 

 so as to convert this hitherto non-vascular membrane into one which is richly 

 supplied with blood-vessels. The chorion has grown in the form of ramified 

 viili into the substance of the uterine mucous membrane or decidua even 

 before this advent of the vascular tissue of the allantois, but the chorionic villi 

 now receive blood-vessels and thus become vascularized, the interchanges between 

 the foetal and maternal vascular systems, which are afterwards confined to one region 

 only of the chorion and decidua that which forms the placenta occurring in the 

 first instance over the whole superficies of the ovum. 



