FCETAL NUTRITION: THE PLACENTA 419 



change in shape of the trophoblast cells and by their pigmen- 

 tation. In the mesoblast of the villi and its vessels no trace is 

 found of any of the formed elements of the embryo trophe, a 

 proof that they undergo further transformation in the tropho- 

 blast after absorption. 



The interchange of oxygen and carbonic dioxide apparently 

 occurs in the labyrinth, as in the cotyledons of the sheep. Here 

 only is the foetal circulation brought into close proximity with 

 circulating maternal blood. Other foetal waste products are 

 probably also got rid of in the labyrinth. Nolf ^ suggests that 

 the excretory products may be responsible for the degeneration 

 of the maternal tissues into a symplasma. 



In how far the other substances necessary for the growth 

 of the embryo are taken up respectively from the circulating 

 blood by purely physical or physiologically selective processes, 

 and from the extravasated blood effusions by direct phagocytosis, 

 is not known. 



Peoboscidea. — In the elephant, the allantois is large and 

 vesicular. Short villi are developed over a large area of the 

 blastodermic vesicle. They lodge in pre-existing depressions 

 in the uterine wall, but the trophoblast is inactive and does not 

 attack the maternal tissues (Assheton ^). Over a zonary area, 

 however, the viUi are much longer, and, penetrating deeply 

 into the maternal tissues, they form a large mass of tissue in 

 the meshes of which maternal blood circulates. Hence the 

 zonary placenta differs from that of Carnivores and resembles 

 that of Insectivores, in which, however, the maternal blood 

 circulates in trophoblastic spaces before the advent of foetal 

 capillaries. 



Though no red blood corpuscles appear to be absorbed as 

 such by the trophoblast, there is evidence of an active absorp- 

 tion of haemoglobin derivatives, the presence of iron compounds 

 being easily demonstrated, especially in the cores of the villi 



' Nolf, "Etude des modifications de la muqueuse uterine pendant la 

 gestation chez le murin," Arch, de Biol., vol. xiv., 1898. 



' Assheton, "The Morphology of the Ungulate Placenta, with Remarks 

 on the Elephant and Hyrax," Phil, Trans. Roy. Soc, London, Ser. B., 

 vol. oxcviii., 1906. 



