FCETAL NUTRITION : THE PLACENTA 513 



transmission of nutriment increases, the trophoblast, which is now 

 represented by the ectodermal covering of the villi, gradually and 

 progressively degenerates. At the end of pregnancy the cytoblast, 

 the mother zone of the plasmodiblast, is reduced to a few scattered 

 groups of cells, while the plasmodial layer itself is no thicker than 

 an endothelium, and may be altogether absent over long stretches 

 of the villi. At this stage it is impossible to believe that the 

 syncytium has any vital functions to perform. Indeed, we know- 

 that it has none, because the foetus, if prematurely born, is able to 

 maintain life without its aid. Hence it seems likely that in the 

 later stages the extra-embryonic ectoderm, though allowing a greater 

 amount of material to pass to the fostus each day, acts merely as a 

 semi-permeable membrane, and has lost all, or nearly all, its 

 physiological activity.^ 



What is the difference in the early stages of pregnancy, when 

 the trophoblast is morphologically well-developed ? We believe that 

 at that time the extra-embryonic ectoderm has less to do with the 

 quantity, and more with the quality, of the material transferred to 

 the new organism. It does not act merely by the physical laws of 

 diffusion and osmosis. At this stage the cells of the ovum have not 

 yet departed widely from a general type, and the active trophoblast 

 would seem to spare the embryonic cells much of the work of the 

 elaboration of the food materials, and thus conserve their energies 

 for their own multiplication and differentiation. As the cells 

 gradually depart further and in different directions from the original 

 type, each cell requires to expend less energy on its own specialisa- 

 tion ; at the same time, tli^ nutritive wants become more varied, and 

 each cell requires to expend more energy on the synthesis of its 

 individual protoplasm. As the duties of selection and anabolism are 

 more and more taken up by the cells themselves, the trophoblast has 

 a less important part to play, and it iindergoes a gradual" process of 

 degeneration.^ 



' Hofbauer's observations on the lisemoglobin metabolism, already quoted 

 (see p. 505), furnish concrete evidence of such a change in the trophoblast. 

 In the first half of pregnancy the syncytium breaks down the maternal hsemo- 

 globin, and subsequently builds it up in part for the foetus. But in the second 

 half, though a greater daily supply of organic iron is required for the formation 

 of haemoglobin and other purposes (see p. 547), the amount of loosely bound 

 iron-compounds in the villi is " extraordinarily small." The only explanation 

 is that the larger molecules of the more firmly combined iron-compounds are 

 not attacked and broken down so strongly by the syncytium, but are passed 

 on to the fcetal circulation. 



2 A similar change occurs in the decidual cells of the rabbit. In the first 

 periods of their existence, they synthesise and store a large quantity of glycogen. 

 In the last week, the cells of the foetal liver assume their glycogenic function, 

 apparently absorbing the carbohydrate from the foetal blood as it returns from 

 the placenta, and the decidual cells degenerate with the loss of their function. 



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