NERVOUS AND ENDOCRINE CONTROL 1 49 



of the male gonad is injected, or if the gland itself is transplanted to such 

 castrates. 



A condition which reveals- the importance of sex hormones in sex dif- 

 ferentiation was discovered by Professor Lillie in his study of the "free- 

 martin," a sterile female calf born as a twin to a male calf. In cattle, when 

 of twin embryos one is male, the other female, the blood vessels in the 

 fetal membranes of the two may fuse in such a way that their blood inter- 

 mingles. The male gonads develop ahead of those of the female with the 

 result that the male sex hormone is the first to pass into the joined cir- 

 culatory systems. It interferes with the growth of the ovary in the female 

 causing sterility and modifying more or less profoundly various of her 

 secondary sexual characters so that they tend to assume the male condi- 

 tion. Various other biologists have shown in several kinds of lower verte- 

 brates that when male and female embryonic or larval forms are grafted 

 together, the sex hormones of one may alter profoundly the sexual sys- 

 tem of the other. 



In the female of the backboned animals, including man, the rhythmical 

 occurrence of ovulation is correlated with rhythmical changes in the 

 secretions of the ovary. In mammals the hormones so far identified with 

 the ovary have been derived from two sources; namely, the follicular 

 fluid which surrounds the Qgg before it is shed from the ovary, and a 

 yellowish mass of cells called the corpus luteimi which come to fill the 

 ruptured follicle after the t^^ has been discharged. The two hormones 

 differ decidedly in function although there appears to be a reciprocal or 

 supplementary relationship between them. The corpus luteum increases 

 in size for a time and then undergoes retrogressive changes and is finally 

 absorbed. The duration of this growth period depends upon whether or 

 not the discharged ovum has been fertilized and is developing in the 

 uterus. If such development is in progress the corpus luteum increases in 

 size and becomes what is known as the corpus luteum of preg7iancy. Its 

 persistence depends upon the length of the gestation period of the animal 

 concerned. If an embryo is not developing in the uterus, the corpus luteum 

 disappears shortly and a new follicle gradually accumulates liquid, projects 

 from the ovarian surface and at the proper interval discharges another 

 ovum. Thus the cycle of ovulation is repeated rhythmically unless inter- 

 rupted by pregnancy. Hormones from the anterior lobe of the pituitary 

 body also have a part in stimulating the ovary to ovulation. 



The corpus luteum hormone seems to have as one of its functions the 

 preparation of the uterine wall for implantation of the fertilized ovum, 

 for if the corpora lutea are destroyed implantation does not occur. How- 

 ever, the uterine wall must first be sensitized by the follicular hormone 

 before the corpus luteum extract is effective. On the other hand, once 

 implantation has taken place, injection of follicular hormone will cause 

 abortion. A product has been isolated from the corpora lutea which relaxes 



