NUTRITION OF THE EMBRYO 1133 



while the lungs are stinted. And since the liver has, as we have 

 already learnt, other and, in the adult at least, even more important 

 labours than excretion, a large portion of the blood it receives 

 is of the best quality: it enters the gland comparatively rich in 

 oxygen, and passes out comparatively poor; while the lungs, which 

 have to be nourished only for their own sake, and are of no use 

 whatever till the child is born and respiration has begun, must be 

 content with the poorest fare with the crumbs that fall from the 

 table of fcetal nutrition. The full-fed cephalic end of the embryo 

 grows far more rapidly than the half-starved inferior extremities, 

 and the head of the new-born child is large in proportion to the rest 

 of the body. 



Metabolism of the Embryo. There are some other points in the 

 physiology of intra-uterine life which call for remark; and, to sum 

 up in a few words the grand distinction between foetal and adult 

 life, we may say that growth is the keynote of the former, work 

 (functional activity) of the latter. Thus, the muscles at an early 

 period in their development, long before any glycogen can be 

 found in the liver, become the seat of an accumulation of glycogen, 

 which, since it cannot be used up in contraction as in the adult 

 muscles, seems to be intimately connected with their own growth, 

 and perhaps also with the growth of other tissues. It is true 

 that the fostal tissues as a whole, including the muscles, are not 

 richer, as used to be taught, but poorer in glycogen than adult 

 tissues, and therefore the old doctrine that the foetal glycogen fulfils 

 a special ' formative ' function in the development of the tissues, 

 has lost its experimental basis. Nevertheless, there is a paral- 

 lelism between the growth of the foetus and its glycogen content. 

 In cases where the growth of the foetus has been spontaneously 

 arrested, the percentage amount of glycogen in its organs has been 

 found to be diminished out of proportion to the diminution in weight. 

 A similar retardation of development can be produced by repeatedly 

 injecting phloridzin into the mother, and thus reducing the glycogen 

 store of the foetus (Lochead and Cramer). Probably, then, the foetal 

 glycogen assists the growth of the embryo, which is known to be 

 accompanied by an intense carbo-hydrate metabolism, by furnishing 

 a store of easily oxidized material for the nutrition of the developing 

 tissues. When the muscles have been formed, their glycogen is 

 still consumed in growth, and their functional powers lie dormant, 

 but for the infrequent and feeble movements, generally regarded as 

 reflex, but possibly to some extent originated in the cerebral cortex, 

 which give the mother the sensation of ' quickening.' It is only 

 late in development that the embryonic liver takes on its glycogenic 

 function. In the earlier stages it is entirely free from glycogen. It 

 is an interesting illustration of that exact adaptation of means to 

 ends which so constantly impresses the investigator of the animal 



