388 THE NUTRITION OF THE EMBRYO. [BOOK iv. 



one and the same time on the material passing from the mother to 

 the foetus influences comparable not only with those exerted by the 

 walls of the alimentary canal but also with those subsequently 

 exerted by the hepatic cells on the material which passes by 

 way of the portal vein from the intestines to the right side 

 of the heart. Again the very phrase " uterine milk " suggests 

 that the placenta epithelial cells exercise a secretory and me- 

 tabolic influence comparable to that of the mammary gland. But 

 how far these analogies are false or true future research must 

 determine; and putting aside for a while the special problems 

 thus suggested we may, in a broad way, say that the foetus lives 

 on the blood of its mother, very much in the same way that all 

 the tissues of any animal live on the blood of the body of which 

 they are the parts. 



959. For a long time all the embryonic tissues are ' proto- 

 plasmic' in character; that is to say, the gradually differentiating 

 elements of the several tissues remain still embedded in un- 

 differentiated material; 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 elabo- 

 rated 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 substance, 

 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. 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 in the foetal body. 



960. Concerning the rise and development of the functional 



