664 



TEXT-BOOK OF PHYSIOLOGY. 



which these vessels give rise come into close relation with and absorb 

 the food material, after which it is carried by veins to the heart, 

 by which it is distributed to all parts of the embryo. These vessels 

 are collectively known as the omphalo-mesenteric arteries and veins. 

 This primitive or vitelline circulation is of short duration in mam- 

 mals, as the nutritive material in the vesicle is small in amount and 

 is soon exhausted. In birds, however, it is of primary importance. 

 The main supply of nutritive material, however, is derived from 

 the mother by means of a highly developed and specialized organ 

 The Placenta. Of all the embryonic structures the placenta 

 is the most important. It is formed by the end of the third month, 



after which it gradually 

 increases in size up to the 

 end of the eighth month, 

 by which time it is fully 

 developed. It then mea- 

 sures from 1 8 to 24 cm. 

 in diameter and weighs 

 from 400 to 600 grams. 

 It is most frequently situ- 

 ated at the upper and 

 back part of the uterine 

 cavity. Though exceed- 

 ingly complex in structure 

 it consists essentially of 

 two portions, a fetal and a 

 maternal. 



The jetal portion con- 

 sists primarily of those villi 

 on the" chorion in relation 

 w ith the decidua serotina. 



ft- ctrnrtnrpc <rraHn 

 inese Structures gradu- 



ally increase in size and 

 number, and receive the 



ultimate branches of the umbilical arteries. The maternal portion 

 consists primarily of the decidua serotina. As gestation advances the 

 chorionic villi rapidly increase in size and number, and receive the 

 branches of the umbilical arteries. At the same time the decidua 

 serotina becomes hypertrophied and vascular. With the continued 

 growth and development of these two structures they gradually fuse 

 together and finally become inseparable. In accordance with the 

 needs of the embryo, the decidua serotina and its contained blood- 

 vessels undergo certain histologic changes which result in the forma- 

 tion of large cavities, sinuses, or lakes, into which the blood of the 

 uterine vessels is emptied. Coincidently the villi of the chorion grow 



FIG. 331. HUMAN EMBRYO, WITH AMNION AND 

 ALLANTOIS, IN THE THIRD WEEK. There 

 are as yet no limbs; the embryo and its 

 appendages are surrounded by the tufted 



chorion. (Haeckei.) 



