THE EGG 



Even in these animals, however^ the uterus is double in its 

 embryonic development. In Rhesus monkeys (Fig. 10, F) and 

 in human infants there is a little notch at the top of the 

 organ, marking the last trace of the doubling. In the other 

 species shown in Fig. 10, namely echidna (A), opossum 

 (B), dog (D), and mare {E) we find various degrees of 

 fusion of the two horns. Sometimes in women the process 

 of fusion fails to be completed, leaving a bicornuate (two- 

 horned) uterus which under certain circumstances may puzzle 

 the gynecologists or even be mistaken for a tumor. In a 

 general way it may be said that animals which bear a single 

 infant, or twins, at one time, have one-chambered uteri or 

 short uterine horns, while those that bear multiple litters 

 have well-separated horns. I hardly know which of these 

 types offers the most striking picture, late in pregnancy 

 when the uterus reaches its largest dimensions — the human, 

 for example, with its infant ensconced in a single huge cham- 

 ber, the cow with an unborn calf in one enormous sac with a 

 little empty horn beside it, or a sow with two long uterine 

 horns each distended like two great strings of sausage, with 

 five to eight 12-inch pigs in each link. 



The lining of the uterus. The longest act of the drama of 

 reproduction is played in the cavity of the uterus. From the 

 first week after the egg is shed from the ovary, until the day 

 of birth, the infant knows no other environment, and depends 

 absolutely upon the reactions of growth and chemical ex- 

 change that take place in these walls that shut it off from 

 everything else in the world. Here occur, as we shall see, some 

 of the most remarkable and important interactions of the 

 hormones of the ovary and the tissues that guard the embryo, 

 and here is the seat of the process of menstruation, strange 

 phenomenon that is an outward sign of human participation 

 in the cosmic tides. 



Because the inner layer or lining of the uterus and what 

 goes on in it will occupy much of this book, we may as well 



{ 61 } 



