386 THE NUTRITION OF THE EMBRYO. [BOOK iv. 



membrane just spoken of. The fluid is also over the rest of the 

 internal surface of the uterus, in close proximity to the blood ves- 

 sels of the maternal decidua, and indeed in the later stages, when 

 the decidua apart from the placenta has largely retrograded, to the 

 blood vessels of the uterine mucous membrane. The conditions 

 therefore are favourable for the transudation of material from the 

 blood of the mother into the amniotic cavity ; and we have experi- 

 mental evidence that not only water but various substances may 

 pass in this way from the one to the other. If indigo-carmine 

 (416) be injected into the veins of the mother, none passes by the 

 umbilical vein into the tissues of the foetus ; these remain wholly 

 uncoloured. Yet the amniotic fluid becomes deeply tinged with 

 the pigment, which obviously must have passed directly from the 

 mother into the amniotic cavity. Hence we may conclude that 

 though the amniotic fluid is at first derived exclusively from the 

 foetus, and during the whole time is partly derived from the same 

 source, it is also, and especially in the later stages, largely derived 

 by direct transudation from the mother. 



Into this amniotic space the passages of the foetus, the mouth, 

 anus, &c. open, and it serves as we shall see as a repository for the 

 excretions of the foetus. Into it is discharged such urine as the 

 foetus secretes, into it are shed the foetal epidermic scales, and ap- 

 pendages such as hairs, and into it may be discharged the contents 

 of the alimentary canal, known as the meconium. Now, hairs, 

 epidermic scales, in the case of hoofed mammals portions of shed 

 hoofs, and at times meconium have been found in the foetal 

 stomach ; they arrived there by the foetus swallowing the 

 amniotic fluid; we have other evidence that the foetus in the 

 uterus may execute swallowing movements, and if these are 

 executed they must lead to swallowing of the amniotic fluid, since 

 this will pass into the mouth and pharynx whenever the mouth is 

 opened. If these swallowing movements occur frequently, and 

 there is some evidence that they do, nutritive material contained 

 in the fluid and derived directly from the mother, might thus be 

 conveyed to the foetus ; the latter might be nourished by means 

 of the amniotic fluid. But, even making all allowance for any 

 possible nourishment in this way, we may probably regard it as 

 insignificant compared with that which is carried on by the 

 placental and umbilical vessels ; we may assume that the food of 

 the foetus reaches it mainly by passing trom the maternal sinuses 

 into the capillaries of the chorionic villi. 



958. Judging from analogy we may conclude that the food 

 of the foetus consists, like that of the adult, of proteids, fats, 

 carbohydrates and salts conveyed in water. In attempting to 

 understand how these materials pass from the blood of the 

 maternal sinus to the blood of the foetal villus, we have to face 

 problems of the same kind as those which we met with in con- 

 sidering absorption from the alimentary canal ( 312). 



