484 THE PHYSIOLOGY OF REPRODUCTION 



lacunae, and in the amount of maternal blood in contact 

 with it. 



The above description does not, however, fit the placenta of 

 Ungulates, for in them the trophoblast is not permeated by 

 maternal blood. If the Insectivore placenta represents the 

 primitive type, or is nearer to it than any other at present 

 existing, we must assume that the Ungulate placenta, differing 

 more widely from the original type, has lost this characteristic. 

 Further, the placenta of the pig must have undergone a greater 

 degree of modification than that of the sheep. In other words, 

 the old ideas of placental evolution, based on the researches of 

 Turner and others, must be literally reversed. The Primates 

 must stand with the Insectivores near the primitive type, while 

 the sheep and pig are near the opposite end, where some of 

 the Didelphia are placed. Such considerations as these must 

 inevitably come up for discussion in all future investigations. 



B. The Nature of the Trophoblastic Activity 



During the period of gestation, the mother organism is 

 concerned with the provision of material for the growth and 

 development of the fertilised ovum and the new-born young. 

 Does the material provided for the ovum, and secured for it by 

 the trophoblast, come from the maternal tissues or from the 

 food supply ? There is no doubt that in insufficient nutrition 

 the foetus draws on the tissues of the mother (Jagerroos 1 ), and a 

 study of comparative placentation goes far to prove that this is 

 a normal process in some orders. It is obvious that such occurs 

 in the earliest stages. In all orders, before fixation of the 

 blastocyst to the uterine mucosa, the degenerating ovarian cells 

 which surround the extruded ovum form a store of nutriment. 

 In some animals, however, such as the opossum, in which no 

 attachment of the blastocyst can be said to occur, and the sheep, 

 in which the attachment is long delayed, this nutriment is 

 added to by the secretion of the uterine glands. In the so- 

 called Deciduate orders, fixation is effected by a phagocytic 

 or chemical action on the part of the trophoblast, and the 



1 Jagerroos, " Der Eiweiss-, Phosphor-, und Salzumsatz wahrend der 

 Graviditat," Arch. f. Gyrak., vol. IxviL 



