FtETAL NUTRITION: THE PLACENTA 441 



around them undergoes fatty degeneration, and in their interior 

 may be seen remnants of connective tissue and endothelial 

 cells and fat-globules. Many capillaries are ruptured, and 

 red and white blood corpuscles are also absorbed. Such an 

 absorption of maternal tissue by the giant-cells leads to 

 an increase in the size of the implantation cavity and a 

 thinning of its wall (Disse). In spite of their abundant 

 supply of nutriment, their hfe-history is short. No cell- 

 divisions occur, and soon they degenerate. Their contents are 

 absorbed by the trophoblast, and their protoplasm shrinks to 

 form a rim around the nucleus. Later still their remnants 

 are also absorbed. 



The allantois in the mouse is a solid mass of mesoderm with 

 no entodermal cavity. Growing out from the posterior end of 

 the embryo, it projects into the extra-embryonic coelom, and on 

 the eleventh day fuses with the mesoblast of the ectoplacental 

 cone. After this the ovum again becomes spherical. The circu- 

 lation in the decidua reflexa diminishes, and gradually more 

 and more of the nutriment is conveyed to the embryo by the 

 allantoic vessels. At the same time the allantoic trophoblast 

 increases in thiclaiess, and its lacunae become more numerous 

 and comphcated. Into its mass, in which the circulation of 

 maternal blood is now estabhshed, the vascular mesoblast pro- 

 jects at intervals, and breaks it up into segments. The glands 

 take no part in the formation of the placenta. Their ducts do 

 not even act as guides to the advancing edge of the trophoblast, 

 as in the rabbit. They are completely displaced by the rapid 

 formation of decidual tissue, and their remnants are absorbed 

 by the giant-cells. Hence the embryotrophe contains no 

 glandular secretion. 



At this time the nutritional conditions are essentially the 

 same as in the rabbit. The trophoblast shows two layers, 

 plasmodiblast and cytoblast, which intervene, along with 

 mesoblastic cells and the walls of the villous capillaries, between 

 the two blood-streams. The subsequent changes are aU in the 

 way of producing an increased surface of contact with maternal 

 blood, and lessening the thickness of tissue between it and the 

 foetal circulation. 



In the mouse the decidual cells contain glycogen. According 



