488 THE PHYSIOLOGY OF REPRODUCTION 



the energy generated in it and by it from the combustion of 

 substances supplied by the mother, and to a series of active 

 metabolic changes by means of which these substances are 

 transformed into living protoplasm. Whether the nutritive 

 materials are derived from the food or tissues of the mother is 

 of secondary importance. What is essential is that the fertilised 

 ovum obtains certain organic and inorganic compounds and a 

 supply of oxygen to carry out its work of organisation, just as 

 in the first period of extra-uterine life the growth and develop- 

 ment of the new being progress by its own activities, so long 

 as it is furnished with the proper materials. 



The special organ of embryonic nutrition is the trophoblast, 

 and evidences of its katabolic activities have been described in 

 various orders of Mammals. But in addition to procuring 

 fixation of the blastocyst to the uterine mucosa, and absorbing 

 and katabolising the food for itself and the embryonic portion 

 of the ovum, it seems also to possess anabolic functions, at 

 least in the earlier periods of pregnancy. Already developed 

 in the blastocyst stage, it is active and functional for a con- 

 siderable time. But in the later stages, it exhibits in all orders 

 of Mammals a degree of morphological degeneration which is 

 incompatible with the maintenance of its early physiological 

 activity. It is further to be noted that its condition varies 

 inversely with the food requirements of the embryo. When the 

 daily requirements for the new organism are almost infinitesimal, 

 the trophoblast is well developed. But as the daily transmission 

 of nutriment increases, the trophoblast, which is now repre- 

 sented by the ectodermal covering of the villi, gradually and 

 progressively degenerates. At the end of pregnancy the cyto- 

 blast, 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 pre- 

 maturely 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 foetus each day, acts merely as a semi-permeable 



