REPRODUCTION IN MAN 1363 



severing all its normal nervous connections, and yet it enlarged as 

 usual during a subsequent pregnancy. It has been shown (Starling 

 and Lane-Claypon) that a somewhat similar hypertrophy may be 

 found in virgin rabbits after the injection of watery extracts made from 

 foetal rabbits. It may therefore be concluded that the chief factor in 

 exciting the growth of the mammary glands during pregnancy is some 

 chemical substance a hormone, produced in the foetus and trans- 

 mitted through the placenta to the maternal blood-stream. This 

 foetal hormone is not the only factor involved in the growth 

 of the mammary glands. Ancel and Bouin have brought forward 

 evidence that the corpus luteum the tissue produced in the ovary as 

 a result of the discharge of an ovum is intimately concerned with the 

 growth of the mammary glands, and may indeed cause a certain degree 

 of hypertrophy of these glands in the entire absence of any product of 

 conception within the uterus.* The limited growth of the glands, which 

 occurs at puberty, can certainly not be ascribed to the presence of a 

 foetus in the uterus, and must be connected with the growth of ripe ova, 

 or, as suggested by the two French authors, with the growth of the 

 tissue of the corpus luteum, resulting from the discharge of ova. 



There seem also to be obscure relationships between the activity of 

 the sexual organs and that of certain so-called ductless glands. Thus 

 castration at an early age leads to persistence of the thymus gland, 

 whereas normally this gland atrophies just before the sexual organs 

 commence their functional activity. The existence of a connection 

 between the thyroid and the ovaries has been a popular belief for 2000 

 years. In many individuals the thyroid is perceptibly enlarged at 

 each menstrual period. On the other hand, extirpation of the thyroid 

 before puberty brings about, among the other signs of cretinism, failure 

 of development of the ovaries, so that puberty is delayed partially or 

 completely. 



We must thus regard the germ-cells not only as representing the cells 

 from which the individuals of the new generation may be developed, 

 but also as concerned in the formation of chemical substances which, 

 discharged into their hosts, affect many or all of the functions of the 



* According to Ancel and Bouin, in the rabbit discharge of an ovum and 

 formation of a corpus luteum only occur as a result of copulation. The same 

 effect may be produced by artificial rupture of a ripe follicle. Whenever this 

 occurs there is a development of the mammary glands. If no impregnation has 

 taken place (e.g. if the buck has been sterilised by ligature of the vas deferens), 

 the glands develop for fourteen days and then begin to atrophy. This period 

 corresponds to the period of active growth of the corpus luteum. The continued 

 growth during the latter half of pregnancy these authors ascribe to the production 

 of another hormone by a special glandular tissue (* myometrial gland ') which 

 makes its appearance about the fourteenth day in the wall of the uterus, at the 

 site of implantation of the placenta, and lasts until the end of pregnancy. 



