DEVELOPMENT 629 



In the deep layer of the mucosa under the placenta the 

 connective tissue cells enlarge and form a thick mass of 

 decidual cells. Possibly these are protective, preventing the 

 enzymes or other products of foetal metabolism from invading 

 the mother, 



A new stage in the physiological relationship of the foetus 

 to the mother is now established. It is no longer a parasite 

 but rather a guest which shares with the mother the supply of 

 nourishment in the maternal food. The placenta becomes (1) 

 the foetal lung, giving the embryo the necessary oxygen and 

 getting rid of the waste carbon dioxide, (2) The foetal 

 alimentary canal supplying the necessary material for growth 

 and development ; and (3) the foetal kidney through which the 

 waste nitrogenous constituents are thrown off. 



When this stage of gestation is reached, it is generally found 

 that the maternal body shows the same increased power of 

 storing the constituents of the food as is seen during the period of 

 growth. The pregnant animal stores material like the growing 

 young, generally taking just what is required to satisfy the 

 normal growth of the foetus, but frequently retaining more and 

 storing it in its body. Hence the practice of having cows 

 rendered pregnant in feeding them for market. 



Carbohydrates are early stored as glycogen in the cells of the 

 maternal placenta, and, since no glycogen is found in the foetus 

 till much later, it must be taken up by the chorionic villi as 

 sugar. Whether the diastase for this conversion is formed by 

 the mother or by the foetus is not known. 



Fats appear earlier in the chorionic villi than in the 

 maternal placenta, and there is no evidence that fats stained 

 with Sudan III. are passed to the foetus. Probably the foetal fats 

 are formed from carbohydrates. 



Iron also appears earlier in the foetal placenta than in the 

 maternal, and it is probably taken up from the hsemolysed 

 maternal blood (p. 626). 



As regards the passage of proteins nothing is known with 

 any certainty. It may be that the proteins of the maternal 

 blood are passed to the foetus unchanged. Since amino-acids 

 are found in the fcetal blood it has been argued that the pro- 

 teins may be digested to this condition before passing to 

 the foetus. But it must be recognised that possibly these 



