504 THE PHYSIOLOGY OF REPRODUCTION 



grown animals may result in a negative nitrogen balance. But 

 in every other instance growth is associated, not with a loss, 

 but with a retention of nitrogen. The conditions are too com- 

 plex to admit of such a simple explanation. 



With the onset of pregnancy certain parts of a grown organism 

 uterus, mammae, &c. are suddenly stimulated to growth. 

 At the same time a new organism is to undergo development. 

 Concurrently with these phenomena of grow h, a negative nitrogen 

 balance occurs in the first half of prf gnancy ; that is to say, 

 pregnancy produces a change in the protein metabolism. In 

 the second hah* of pregnancy, growth is associated with a re- 

 tention of nitrogen by the mother. In other words, the con- 

 ditions which come into existence at the b ginning of gestation, 

 and lead to a negative nitrogen balance, alter during its course. 



That growth proceeds along with a loss of nitrogen is note- 

 worthy. It is at variance with the conditions found in all 

 other instances of physiological growth, with which, however, 

 the growth of certain organs in pregnancy cannot be compared, 

 since it alone occurs in a grown mammal. It is conceivable 

 that this phenomenon in itself is sufficient to disturb the protein 

 metabolism. As was previously mentioned, it is not known 

 whether the stimulus for the changes during pregnancy is derived 

 from the fertilised ovum itself, the corpus luteum, or some other 

 less obvious factor. It has been experimentally proved that 

 the corpus luteum is essential in the early, but not in the later 

 stages. 1 Its development dates from the period of conception, 

 and its activity lasts until the middle of pregnancy, after which 

 it undergoes structural and functional degeneration. Hence 

 the period of its activity corresponds with the period of the 

 negative nitrogen balance in the mother. Hagemann's observa- 

 tion of a similar nitrogen balance at a certain phase of the 

 cestrous cycle when corpora lutea are also developed, though no 

 ovum is fertilised, would seem to favour the view that the loss 

 of nitrogen is in some way connected with the changes in the 

 ovary. 



Like the corpus luteum, the trophoblast undergoes a marked 

 change during pregnancy. In the early stages it forms the 

 special organ of nutrition for the embryo, and in addition to 

 1 Marshall and Jolly, toe. tit. (see pp. 336-345, 351, and 491). 



