248 EMBRYOLOGY. 



latter (fig. 141 am}) up to its place of attachment to the chorion. 

 The evagination of the entodermio layer meantime becomes elongated 

 into the narrow allantoic duct ; the more voluminous connective- 

 tissue gro-wiih carries with it the umbilical blood-vessels to the 

 chorion, then spreads itself out on the inner surface of the latter 

 in the well-known manner, and penetrates into the villi of the 

 serosa. 



The allantois, therefore, in its development, instead of growing 

 out free to the serosa, makes use of the already existing connection 

 between the latter and the embryo established by the pennant-like 

 elongation of the amnion (am}). But this mode of development 

 perhaps results from the fact that the posterior end of the embryo 

 in Man, as fig. 141 shows, is closely attached to the serosa at the 

 place of the amniotic suture, whereby the allantois has only a short 

 distance to grow in order to reach the serosa. 



Finally, the early appearance of the allantois will become intel- 

 ligible to us, if we remind ourselves that organs of great physiological 

 importance have in general the tendency to an accelerated develop- 

 ment, and that in the series of Mammals the provisions for the 

 nutrition of the embryo by means of a placenta have become more 

 and more complete. 



While there is still much obscurity about the first stages of Man's 

 development, we possess more satisfactory insight into the changes 

 which the embryonic membranes in Man undergo from the third 

 week onward. 



From this point forward we shall examine each separate embryonic 

 membrane by itself : first the structures that are developed from 

 the blastodermic vesicle — (1) the chorion, (2) the amnion, (3) the 

 yolk-sac ; then (4) the deciduse which are produced by the mucous 

 membrane of the uterus ; and finally (5) the after-birth (placenta) 

 and (6) the umbilical cord. 



1. The Chorion. 



During the first weeks of pregnancy the whole surface of the 

 chorion is covered with villi (fig. 132^, p. 226, and fig. 140), and 

 provided with terminal branches of the umbilical blood-vessels. After 

 its growth has proceeded for a time uniformly, there begin to appear — 

 from the beginning of the third month onward — difierences between 

 the part which lies directly against the wall of the uterus that is 

 destined to become the decidua serotina and the remaining greater 



