THE FETAL MEMBRANES AND THE PLACENTA. 



The maintenance of the intra-uterine existence of the fetus, 

 and its development to a degree which will enable it to maintain 

 a more or less independent existence at the time of birth, requires 

 that effective means be established for the exchange of nutritive 

 and waste materials between the mother and her young, not 

 alone for the basic purposes of nutrition and excretion, but also 

 that the embryo may be moored or fixed at a given point in the 

 maternal organ, where its position can be maintained throughout 

 the duration of pregnancy in such a manner as to best protect 

 and insure its life and normal growth. To this end there are 

 formed three structures from the blastodermic vesicle, which 

 undergo changes to finally constitute the fetal membranes and 

 placenta. 



I. The Vitelline, or Yolk Sac. 



When the embryo commences to develop, the embr\'onic area 

 folds inward at its borders, leading to a constriction between this 

 area and that part of the blastodermic vesicle which lies beyond, 

 and they finally become separated except by a narrow neck, the 

 vitelline duct, while, beyond, there exists the comparatively 

 large vitelline or yolk sac. This vitelline, or yolk, sac plays but 

 a minor part in the development of the embryo and tends to 

 more or less completely disappear, according to species, while in 

 some it persists as an embryonal vestige to the time of birth. In 

 the mare, the yolk sac is very inconspicuous early in fetal life 

 and disappears almost completely at a very early stage. In 

 Fig. 74 this general plan is suggested at the twenty-eight day of 

 pregnancy, while in Fig. 75 it is indicated that, at five months, 

 it has well nigh disappeared. 



In the sheep, as shown in Fig. 76, it assumes a wholly differ- 

 ent form and undergoes great elongation. In other of our 

 domestic animals there are variations in the form which this sac 

 assumes, but, in each alike, it is of apparently little importance 

 except during the very earliest stages of embryonic life. 



2. The Amnion. 

 The amnion commences to form as soon as segmentation has 

 been completed and the ovum has passed through the oviduct 

 and reached the uterus. This occurs at about the 13th or 14th 



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