Sio THE PHYSIOLOGY OF REPRODUCTION 



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 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 contains 

 cellular elements of maternal tissue, particularly leucocytes and 

 glandular epithelium, which are ingested and dissolved by the 

 trophoblast during the whole period of gestation. In addition, 

 extravasations of maternal blood or individual corpuscles occur in, all, 

 and the erythrocytes are also taken up and dissolved. In such orders 

 maternal tissue elements are normally used for the foetus 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 variotis 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 

 Chapter XL, p. 521), 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 

 transferred ready-made from the mother? The limitations of 

 biological chemistry force us to approach this problem indirectly. 

 Differential analyses of special constituents of the blood, as the, pro- 

 teins, 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 

 anabolic processes which are . carried out by the ovum itself. There 

 is no reason to believe that the mammalian ovum, after acquiring the 

 property of intra-uterine development, has lost its metabolic activity.^ 



^ The Carnivora, in which the trophoblast is not in contact with circulating 

 maternal blood, occupy a special position among the Deciduata, and are not 

 considered above. 



^ If Hubrecht's view is correct, that the mammalian ovum is older than 

 the ovum of birds (see "Early Ontogenetic Phenomena in Mammals," ^ar. 

 Jour. Micr. Science, 1908), the sentence ought to read : "There is no reason to 

 believe that the fertilised ovum of birds acquired its metabolic activities only 

 after the loss of viviparity." 



