458 THE PHYSIOLOGY OF REPRODUCTION 



twentieth days (Fig. 130). From the sixteenth day they are also 

 found in an increasing number of the decidual cells which lie close 

 to the foetal placenta ; after the twenty-fourth day, when the cells 

 degenerate, the granules are no longer discrete, but there are irregular 

 blue-black patches up to the end of pregnancy. 



Such isolated data cannot be accurately interpreted. The fact 

 that the deposits in the three tissues are always situated in apposition 

 to each other speaks for their absorption by the fcetal tissues ; on the 

 other hand, a very small number of granules are present in the 

 trophoblast, and only for a few days. It is possible that organic 

 iron compounds, not shown by the hematoxylin stain, are absorbed 

 and broken up, and later appear as granules in the mesoderm. Their 

 further course to the foetal liver, in which they are stored, has not 

 been traced. It is to be noted that the iron compounds are not only 

 derived from haemoglobin. They may also represent degeneration 

 products of the nucleoproteins. 



Hubrecht^ has suggested that erythrocytes may be -manufactured 

 in the decidual cells, and their iron-containing granules may thus be 

 utilised (see p. 518). 



Fat Metabolism 



Regarding the presence of fat in the placenta of a rabbit, a few 

 observations have been made by Eden,^ Maxim ow, and Masius. 

 Chipman has investigated the subject in greater detail, but he draws 

 no conclusions from the histological data. In reality, the study of 

 fat in the placenta is rendered difficult by its occurrence both as an 

 infiltration and in the degeneration of cells. 



Fat is found in the foetal viscera, liver, heart, and mid-gut, before 

 the allantoic circulation is established. At this time, the tenth day, 

 the vitelline circulation is at its height, and the fat probably reaches 

 the embryo by its vessels, as it is also found in the hypoblast of the 

 area vasculosa. It may be derived from the absorption by the 

 trophoblast of fat droplets contained in the giant-cells of the peri- 

 placental folds. As the vitelline circulation diminishes, the fat 

 disappears from the embryonic viscera, and does not reappear till 

 four or five days after the establishment of the allantoic circulation. 

 During this interval fat is present in the extra-placental wall of the 

 blastocyst, but it probably arises by a degeneration of its cellular 

 protoplasm. 



In the foetal placenta, fat is never found in the mesoblast or 

 capillary walls, but it occurs in the trophoblast, especially where it 



' Hubrecht, " Ueber die Entwioklung der Placenta von Tarsius und Tupaja," 

 Internat. Gongr. of Zool., Cambridge, 1898. 



2 JEden, "The OceuiTence of Nutritive Fat in the Human Placenta," Proc. 

 Roy. Soc. London, vol. Ix., 1896. 



