128 CYCLIC CHANGES IN THE OVARIES AND UTERUS OF THE SOW, ETC. 



twelfth day the vesicles are 10 to 12 mm. in length by about 3 mm. in diameter 

 at their widest part; and by the fourteenth day they are 20 to 30 cm. long, with 

 embryonic areas showing the neural groove well developed and possessing 1 to 5 

 somites. At 17 or 18 days the uterine cavity is completely filled by the embryonic 

 envelopes, which, when their accordion-like folds are eliminated by tension, are 

 found to have reached their full length of 30 to 40 cm. 



It is important to remember that the relations between maternal and fetal 

 tissues are of the simplest character in this species. No decidua is formed from 

 the uterine mucosa, which retains during gestation a structure not widely different 

 from that of the non-pregnant period, and the chorion simply becomes applied, 

 over practically its whole surface, to the epithelium of the uterus (fig. 33, pi. 4). 

 Moreover, the chorion itself remains a simple one-layered membrane, against the 

 inner surface of which, after the third week, the vascular allantois is intimately 

 applied. Nutritive substances passing from mother to embryo must then traverse 

 both the uterine epithelium and the columnar chorionic epithelium in order to enter 

 the fetal (allantoic) vessels. At all periods of pregnancy the chorion is readily 

 detached by gentle traction and at parturition the fetal membranes separate from 

 the uterine wall without lesion or hemorrhage, leaving an intact surface an 

 arrangement which seems excellently adapted to minimize the strain of giving 

 birth to litters sometimes as numerous as 20 or more. 



In view of the diffuse non-deciduate placentation, it is not possible to define 

 sharply the time of implantation. We can say only that after about the tenth day 

 growth of the vesicle is so extensive that it must then be considered dependent upon 

 maternal nutrition rather than upon its own resources; by the thirteenth day, if 

 not earlier, its position is fixed ; by the fifteenth the chorion has come into relation 

 with a large part of the uterine mucosa; while during the third week the allantois 

 begins to line the inner surface of the chorion and hence brings fetal and maternal 

 tissues into their definite relationship. It is the period from the tenth to the 

 fifteenth day that is most nearly comparable to the much more precisely limitable 

 act of implantation which occurs in animals like the rabbit and guinea-pig, and 

 presumably in man. 



