FCETAL MEMBRANES. 107 



to form the yolk stalk which connects the yolk sac with the ventral side of the 

 embryonic body (Fig. 100, 5). 



While the processes just described have been taking place, an evagination ap- 

 pears pushing out from the ventral side of the caudal end of the gut (Fig. 100, 4). 

 This evagination grows out into the extraembryonic body cavity (exo- 

 coelom), pushing before it the visceral layer of mesoderm, thus giving rise to a 

 thin-walled sac which communicates w r ith the gut the allantois (Fig. 100, 5). 

 At this stage the embryonic body, with its surrounding amnion and appended 

 yolk sac and allantois, lies within the large vesicle formed by the chorion. Up 

 to this point the development resembles that in the chick. 



In succeeding stages a new connection is established between the embryo and 

 the chorion in the following manner : The amnion enlarges and fills relatively 

 more of the cavity within the chorion, while the yolk sac becomes smaller and 

 the yolk stalk much attenuated (Fig. 100, 6). At the same time the allantois 

 also becomes attenuated and its distal end comes in contact with the chorion 

 (Fig. 100, 6). The growth of the amnion results in the pushing together of the 

 attenuated yolk stalk and allantois so that they lie parallel to each other (Fig. 

 100, 6), and are together invested by a portion of the amnion. As already 

 described, both yolk stalk and allantois are composed of entoderm and 

 mesoderm while the amnion is composed of mesoderm and ectoderm. Con- 

 sequently when the three structures come together and fuse, there is formed 

 a mass of mesoderm which contains the entoderm of the yolk stalk or vitelline 

 duct and the entoderm of the allantois or allantoic duct, and which is sur- 

 rounded by the ectoderm of the amnion. The fusion of these three structures 

 in this region thus produces a slender cord of tissue which forms the union 

 between the embryo and the chorion and which is known as the umbilical cord 

 (Fig. 100, 6). 



In Mammals the yolk sac contains little or no yolk and consequently can 

 furnish but little nutriment for the embryo; but the union of the allantois with 

 the chorion, mentioned in the preceding paragraph, allows the allantoic blood 

 vessels to come into connection with the chorion. And since in Mammals the 

 chorion is the means of establishing the communication between the embryo 

 and the mother, the allantoic (umbilical) vessels assume the function of carrying 

 nutrient materials to the embryo and also of carrying away from the embryo its 

 waste products. (See p. 223.) 



Further Development of the Chorion. 



Up through the stages which have been described the correspondence in the 

 development of the foetal membranes in Reptiles, Birds and Mammals is clear. 

 From now on, the course of development in Mammals becomes more and 

 more divergent. The extensive development of the yolk and yolk sac with its 



