FCETAL NUTRITION: THE PLACENTA 471 



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 all 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 inouse the decidual cells contain glycogen. According to 

 Driessen.i its distribution in the placenta of the white mouse before 

 mid-term is the same as in the rabbit. It is in great abundance in 

 the decidual cells, especially in the boundary layer between the 

 maternal and foetal tissues. No glycogen is found in the maternal 

 endothelium or in the foetal placenta. Jenkinson^ has studied the 

 distribution of glycogen throughout the whole period of gestation in 

 the mouse. It appears first in the cells which overlie the ecto- 

 placenta, and increases in amount till the twelfth day, when the 

 mesoblastic processes are just beginning to project into the tropho- 

 blast. Then the decidual cells are disintegrated and the glycogen 

 granules are mixed with the detritus. Hence the life-history of the 

 maternal glycogenic tissue is shorter than in the rabbit. But in 

 the mouse glycogen again makes its appearance in the trophoblast . 

 which is most directly in contact with the maternal blood, i.e. the 

 part not penetrated by the allantoic capillaries. It lies in oblong 

 ectodermal cells, which gradually encroach on and occupy the 

 space previously occupied by the maternal glycogenic cells down 

 to the muscularis. Here the glycogen remains till the end of 

 gestation.^ 



" There can be no doubt that this tissue holds in reserve a store of 

 food material for the use of the embryo. As sugar the glycogen 

 passes into the maternal vessels and into the lacunse, and so is 

 absorbed by the foetal capillaries. When the glycogen is used up 

 the cells collapse, and their collapse may be a factor in determining 

 the moment of parturition, since it is across this layer that the 

 placenta breaks away. The trophoblastic is much more voluminous 

 than the maternal glycogenic tissue ever was " ( Jenkinson *). 



According to Kolster, a considerable amount of fat appears in the 

 decidua, in which the connective tissue and endothelial cells undergo 

 a fatty degeneration in the proximity of the giant-cells. No observa- 



' Driessen, " Ueber Glykogen in der Placenta," Arch. f. Ch/niik., vol. Ixxxii., 



1907. 



2 Jenkinson, loc. cit. See also Brit. Med. Jour., 1904. 



3 "Whether the' differences in the distribution ; of the placental glycogen 

 in the rabbit and the mouse during the later stages of pregnancy exist in 

 reality, or depend only on differences of interpretation, requires further 

 investigation. 



* Jenkinson, Vertebrate Embryology, Oxford, 1913. 



