676 FCETAL METABOLISM, [BOOK iv. 



life, nutritive materials are conveyed to or excretions conveyed from 

 the growing young. The placenta is remarkable for the great de- 

 velopment of cellular structures, apparently of an epithelial nature, 

 on the border-land between the maternal and foetal elements ; and 

 it has been suggested that these form a temporary digestive and 

 secretory (excretory) organ. But we have no exact knowledge of 

 what actually does take place in these structures. From the coty- 

 ledons of ruminants may be obtained a white creamy-looking fluid, 

 which from many features of its chemical composition might be 

 almost spoken of as a 'uterine milk/ 



Speaking broadly, the foetus lives on the blood of its mother, 

 very much in the same way as all the tissues of any animal live 

 on the blood of the body of which they are the parts. 



For a long time all the embryonic tissues are 'protoplasmic' in 

 character; that is, the gradually differentiating elements of the 

 several tissues remain still embedded, so to speak, in undifferen- 

 tiated protoplasm ; and during this period there must be a general 

 similarity in the metabolism going on in various parts of the body. 

 As differentiation becomes more and more marked, it obviously 

 would be an economical advantage for partially elaborated material 

 to be stored up in various foetal tissues, so as to be ready for 

 immediate use when a demand arose for it, rather than for a 

 special call to be made at each occasion upon the mother for 

 comparatively raw material needing subsequent preparatory changes. 

 Accordingly, we find the tissues of the foetus at a very early period 

 loaded with glycogen. The muscles are especially rich in this 

 substance, but it occurs in other tissues as well. The abundance 

 of it in the former may be explained partly by the fact that they 

 form a very large proportion of the total mass of the foetal body, and 

 partly by the fact that, while during the presence of the glycogen 

 they contain much undifferentiated protoplasm, they are exactly 

 the organs which will ultimately undergo a large amount of 

 differentiation, and therefore need a large amount of material for 

 the metabolism which the differentiation entails. It is not until 

 the later stages of intra-uterine life, at about the fifth month, 

 when it is largely disappearing from the muscles, that the glycogen 

 begins to be deposited in the liver. By this time histological 

 differentiation has advanced largely, and the use of the glycogen 

 to the economy has become that to which it is put in the ordinary 

 life of the animal ; hence we find it deposited in the usual place. 

 Besides being present in the foetal, glycogen is found also in the 

 placental structures; but here probably it is of use, not for the 

 foetus, but for the nutrition and growth of the placental structures 

 themselves. We do not know how much carbohydrate material 

 finds its way into the umbilical vein; and we cannot therefore 

 state what is the source of the foetal glycogen ; but it is at least 

 possible, not to say probable, that it arises, in part at all events, 

 from a splitting up of proteid material. 



