300 LIFE : OUTLINES OF GENERAL BIOLOGY 



covered by Graaf in the seventeenth century and mistaken for the 

 ova themselves. Each foUicle is a nest with a cavity, and the egg- 

 cell lies in the middle of a disc of cells at one side. The cavity 

 • contains a fluid, the follicular fluid, an exudate from the surrounding 

 cells. It may be noted that the egg-cells of mammals (except the 

 egg-laying duckmole and spiny ant-eater) are very small, much 

 smaller than frog's eggs, which are about a tenth of an inch in 

 diameter. The ova of the whale are no larger than "fern's seed"; 

 those of a rabbit have a diameter of about .^jio inch. 



When a follicle which protrudes on the surface of the ovary is 

 ripe, it bursts, and the liberated ovum is caught by the apposed 

 mouth of the oviduct, where fertilisation may take place. The 

 fertilised ovum, beginning to develop, passes down into the uterus, 

 where it is implanted and proceeds to give rise to an embryo. 



But the burst follicle has a remarkable history. The follicle cells 

 begin to grow and develop in an extraordinary way, forming large 

 glandular cells of a yellow or reddish brick colour — the luteal cells. 

 As the result a large vitally active body is formed — the corpus 

 luteum, which protrudes on the surface of the ovary. There is 

 strong reason to believe that this body liberates into the blood 

 certain hormones which have a powerful influence on various parts 

 of the mother's body. 



There seems to be a tendency to overload the corpus luteum with 

 functions in the way of preparing the mother-organism for preg- 

 nancy; and there is an obvious difficulty in the fact that some of 

 the preparatory changes begin before active corpora lutea are 

 present. Hence the importance of recent work by Allen and Doisy 

 (1923}. which goes to show that the follicle cells have a share in 

 the preparation. During the growth of the Graafian follicle the 

 follicle cells appear to secrete a hormone, which passes into the 

 follicular fluid and thence into the blood. Experiments show that 

 this follicular hormone (included in the fluid) can evoke some of 

 the changes which were formerly put to the credit of the corpus 

 luteum. 



Some years ago Dr. Voronoff was impressed by the flabbiness, 

 inferior intelligence, early senility, and relatively short life of 

 Egyptian eunuchs, and attributed these features to the absence 

 of the normal testicular hormones. Similarly he was impressed by 

 the tendency to early decrepitude and the like in castrated 

 mammals. So just as thyroid deficiency in man is counteracted by 

 treatment with preparations made from the thyroid glands of 

 mammals (or nowadays from synthetised thyroxin), Voronoff 

 suggested that the experiment should be made of grafting repro- 

 ductive glands into the ageing organism. The transference of a 

 reproductive organ from one individual to another of the same 

 species is no novelty; thus it was successfully effected twenty-five 



