854 THE PHYSIOLOGY OF REPRODUCTION. 



the maternal organism digests and prepares the food for the embryo > 

 excretes the wastes, regulates the conditions of temperature, etc., 

 as it does for a portion of its own substance, but as the fetus ap- 

 proaches term its tissues and organs begin to assume more of an 

 independent activity, as indeed must be the case in preparation 

 for the sudden change at birth. In this respect, as in all parts of 

 the reproductive process, we meet with regulations whose mechanism 

 is but dimly understood. 



Changes in the Maternal Organism during Pregnancy. 

 The two most distinct effects upon the mother that result from 

 pregnancy are the growth of the uterus and of the mammary gland. 

 The virgin uterus is small and firm, weighing from 30 to 40 gms., 

 while at the end of pregnancy it may weigh as much as 1000 gms. 

 This great increase in material is due partly to the growth of new 

 muscular tissue and partly to an hypertrophy of the muscle already 

 present. In the uterus at term the muscle cells are much longer 

 and larger than in the organ before fertilization. The stimulus 

 that initiates and controls this new growth is seemingly the fertil- 

 ized ovum itself, but the physiological means employed are not 

 comprehended. We know from experiments upon lower animals 

 (Rein) that when all connections with the central nervous system 

 are severed the fetus develops normally and the uterus increases 

 correspondingly in size and weight. The influence of the ovum on 

 the uterus must be exerted, therefore, either through some local 

 nerve centers in the uterus, or, as seems much more probable, 

 through some chemical stimulus which it gives to the organ. The 

 effect of the presence and growth of the ovum on the mammary 

 gland is treated in a separate paragraph below. In addition to 

 these two visible effects it is evident that the growth of the fetus 

 has an important influence on general metabolism and therefore 

 upon the whole maternal organism. This fact is indicated by the 

 marked changes often exhibited in the physical and mental con- 

 dition of the mother. It is shown more precisely by a study of the 

 nutritional changes. Numerous investigations have been made 

 upon this side especially as regards the nitrogen equilibrium. Dur- 

 ing the latter part of pregnancy especially the nitrogen balance is 

 positive, that is, nitrogen is stored as proteid, due doubtless 

 both to the growth of the embryo and the increase in material in 

 the uterus and mammary gland. The proportion of ammonia in 

 the urine increases during pregnancy and especially during labor 

 (Slemmons*), a result which may be due to some interference 

 with the normal functions of the liver. 



Parturition. The fetus "comes to term" usually in the tenth 

 menstrual period after conception, that is, about 280 days after 



* Slemmons, "The Johns Hopkins Hospital Reports," 12, 111, 1904. 



