FCETAL NUTRITION: THE PLACENTA 485 



destroyed maternal tissue again seems to serve as food for the 

 blastocyst. 



After fixation, differences appear in the various orders. In 

 Ruminants a special nutritive secretion, the uterine milk, is 

 elaborated in the inter-cotyledonary areas. This secretion con- 

 tains cellular elements of maternal tissue, particularly leucocytes 

 and glandular epithehum, which are ingested and dissolved by 

 the trophoblast during the whole period of gestation. In 

 addition, extravasations of maternal blood or individual cor- 

 puscles occur in aU, and the erythrocytes are also taken up and 

 dissolved. In such orders maternal tissue elements are normally 

 used for the fcetus throughout pregnancy. 



Among the Deciduata, however, with the exception of the 

 mole, in which the glandular secretion is maintained, the 

 maternal blood may be considered to be the only source of 

 foetal nutriment when the allantoic or chorionic placenta is 

 developed.^ In them the trophoblast resembles a sponge 

 saturated with slowly circulating blood, and its large superficies 

 is admirably adapted for the acquisition of the various materials 

 required for the foetus. In what form do these materials exist 

 in the blood ? Are they simply the substances absorbed from 

 the food by way of the intestine (see also Chap. XI., p. 495), or 

 are they more highly elaborated ? In other words, in the 

 formation of the new organism are the syntheses carried out by 

 the fertilised ovum itself, and it must be remembered that this 

 includes the trophoblast, or are the new tissue-elements trans- 

 ferred ready-made from the mother ? The hmitations of 

 biological chemistry force us to approach this problem indirectly. 

 Differential analyses of special constituents of the blood, as the 

 proteins, in the non-pregnant and pregnant animal are not yet 

 possible. 



In the first place, a brief consideration of the development 

 of the chick embryo is sufficient to prove the high degree of 

 activity vested in the ovum of birds. The special proteins 

 and other tissue-elements are not pre-formed, but are elaborated 

 by a series of katabolic and anabohc processes which are carried 



' The Carniyora, in which the trophoblast is not in contact with circu- 

 lating maternal blood, occupy a special position among the Deciduata, and 

 are not considered above. 



