THE DEVELOPMENT 'OP THE EMBRyO ITSELF. 123 



but, of course, if impregnated in tlie spring, there will be no au- 

 tumn cestrum on account of the prolonged period of gestation 

 in this instance ; and, similarly, in the case of the cow and other 

 animals. 



It is important to recognize that rutting is only the evidence 

 of the maturation of the Graafian follicle within the ovary and 

 of correlated changes. 



In a state of nature — i. e., in the case of wild animals — the 

 male experiences a period of sexual excitement corresponding 

 with an increased activity of the sexual organs and at periods 

 answering to the rutting season of the female. In some species 

 the testes . descend into the scrotum only at this season. This 

 may be observed in the rabbit. But in our domestic animals, as 

 a class, the male, though capable of copulation at all times, is ex- 

 cited only by the presence of a female in season. It is only at 

 such periods that the approach of the male is permitted by the 

 opposite sex. 



THE NUTRITION OP THE OVUM (OOSPERM). 



This will be best understood if it be remembered that the 

 ovum is a cell, undifferentiated in most directions, and thus a 

 sort of amoeboid organism. In the fowl it is known that the 

 cells of the primitive germ devour, amceba-like, the yelk-cells, 

 while in the mammalian oviduct the ovum is surrounded by 

 abundance of proteid, which is doubtless utilized in a somewhat 

 similar fashion, as also in the uterus itself, until the embryonic 

 membranes have formed. To speak of the ovum being nour- 

 ished by diffusion, and especially by osmosis, is an unnecessary 

 assumption, and, as we believe, at variance with fundamental 

 principles; for we doubt much whether any vital process is 

 one of pure osmosis. As soon as the yelk-sac and allantois 

 have been formed, nutriment is derived in great part through 

 the vessel-walls, which, it will be remembered, are differenti- 

 ated from the cells of the mesoblast, and, it may well be as- 

 sumed, have not at this early stage entirely lost their amoeboid 

 character. The blood-vessels certainly have a respiratory func- 

 tion, and suflRce till the more complicated villi are formed. 

 The latter are in the main similar in structure to the villi of the 

 aUmentary tract, and are adapted to being surrounded by sim- 

 ilar structures of maternal origin. Both the maternal crypts 

 and the foetal villi are, though complementary in shape, all but 



