FCETAL NUTRITION: THE PLACENTA 449 



attachment attacks the uterine mucosa more strongly than in the 

 Ungulata. As a result, the maternal tissues, with the exception of 

 the septa containing the placental vessels, disappear down to the 

 middle of the spongy layer, and the tissue which is destroyed serves 

 as pabulum for the developing embryo. Van der Brock ^ suggests 

 that the general oedema of the uterine mucosa may lead, as elsewhere, 

 to its malnutrition and degeneration, and thus it may fall an easy 

 prey to the trophoblast. Others maintain that the degeneration is 

 brought about by a trophoblastic influence, perhaps of the nature of 

 an enzyme action. The result is a transformation of all the elements 

 to a symplasma. In the cat the connective tissue cells may form 

 large decidual cells before their final destruction. 



As in the Indeciduates, there is strong histological evidence 

 that the trophoblast is actively phagocytic, and takes up, as it meets 

 them, the constituents of the pjochorion, and later the degenerated 

 tissues and extravasated blood. In the neighbourhood of the 

 extravasations active absorption is indicated by the change in shape 

 of the trophoblast cells and by their pigmentation. In the mesoblast 

 of the vilh and its vessels no trace is found of any of the formed 

 elements of the embryotrophe, a proof that they undergo further 

 transformation in the trophoblast 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 fcetal 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.^ 



^ Van der Brock, " Die EiMllen und die Placenta von Phoca 

 Petrtis Camper, D. ii. Quoted by Kolster {Ergebn. d. Anat., vol. xvi., 1906). 



^ Nolf, "Etude des modifications de la muqueuse uterine pendant la gestation 

 chez le murin," Aroh. de Biol., vol. xiv., 1896. 



' Cunningham has investigated the fluid and salt interchanges between 

 mother and foetus in cats in the later stages of pregnancy. He injected into the 

 venous system balanced solutions of potassium f errocyanide and iron ammonium 

 citrate, salts which can be precipitated as Prussian bite, and consequently 

 'can be readily followed along the route traversed to their ultimate location. 

 It was found that both maternal and foetal endothelia were easily permeable 

 to the two salts, but foetal ectoderm reacted differently in terms both of 

 permeability and length of time required. In experiments of short duration 

 no trace of either salt was found in the amniotic iiuid, foetal urine, or tissue 

 extract. In those of longer duration sodium ferrocyanide was found in the 

 amniotic fluid and foetal urine, but no trace of iron ammonium citrate was ever 

 detected in any foetal tissues. The placenta in the shorter experiments showed 



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