Chap, ii.] PREGNANCY AND BIRTH. 1129 



and if so how, — to these and other questions which suggest 

 themselves no very satisfactory answer can at present be given. 

 With regard to fat, leaning on the analogy of the conclusion 

 at which (§ 486) we arrived, that in the adult the fat of the 

 food is probably not taken up by the tissues as fat during the 

 nutrition of the tissues by the blood, we may perhaps suppose 

 that the mother does not supply the foetus with fat as such. 

 We have already referred to the significant presence of glyco- 

 gen in the placenta ; and it would almost seem as if the pla- 

 centa exerted at 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 exer- 

 cise a secretory and metabolic 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. 



§ 700. For a long time all the embryonic tissues are ' pro- 

 toplasmic' in character; that is to say, the gradually differen- 

 tiating elements of the several tissues remain still embedded in 

 undifferentiated 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 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 need- 

 ing subsequent preparatory changes. Accordingly, we find the 

 tissues of the foetus at a very early period loaded with glyco- 

 gen. 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 glyco- 

 gen 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 



