THE NUTRITION OF THE EMBRYO. 849 



many features of its chemical composition might almost be 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. 



791. For a long time all the embryonic tissues are " protoplasmic" in 

 character ; that is, the gradually differentiating elements of the several tis- 

 sues remain still imbedded, so to speak, in undifferentiated 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 mate- 

 rial 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 gly- 

 cogen 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 it way into the umbilical vein ; and we cannot, 

 therefore, state what is the source of the fcetal 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. 



792. Concerning the rise and development of the functional activities 

 of the embryo our knowledge is almost a blank. We know scarcely any- 

 thing about the various steps by which the primary fundamental qualities 

 of the protoplasm of the ovum are differentiated into the complex phenom- 

 ena which we have attempted in this book to expound. We can hardly 

 state more than that while muscular contractility becomes early developed, 

 and the heart probably, as in the chick, beats even before the blood-corpus- 

 cles are formed, movements of the foetus do not, in the human subject, 

 become pronounced until after the fifth month ; from that time forward they 

 increase and subsequently become very marked. They are often spoken of 

 as reflex in character ; but only a preconceived bias would prevent them 

 from being regarded as largely automatic. The digestive functions are 

 naturally, in the absence of all food from the alimentary canal, in abeyance. 

 Though pepsin may be found in the gastric membrane at about the fourth 

 month, it is doubtful whether a truly peptic gastric juice is secreted during 

 intra-uterine life ; trypsin appears in the pancreas somewhat later, but an 

 amylolytic ferment cannot be obtained from that organ till after birth. The 

 date, however, at which these several ferments make their appearance in the 

 embryo appears to differ in different animals. The excretory functions of 



54 



