1362 PHYSIOLOGY 



responsible for rut, as well as for menstruation, are independent of any 

 nervous connections between the ovaries and the rest of tlie body, and 

 must therefore be brought about by the circulation in the blood of 

 specific chemical substances produced in the ovaries. According to 

 some authors, the essential factors for the production of these genital 

 hormones are the ' interstitial cells ' found both in the testes and 

 ovaries of various animals. These interstitial cells are not, however, 

 universally present. It has been shown that, by means of the Rontgen 

 rays, it is possible to destroy the germ-cells in either testes or ovaries, so 

 rendering the animal sterile. The interstitial cells, when present, are 

 not destroyed by these rays, yet the effects on the accessory genital 

 organs are stated to be as marked as after complete extirpation of 

 either ovaries or testes. 



The chemical correlations between the ovaries and the other 

 organs concerned in reproduction are perhaps best marked in the 

 changes which attend pregnancy. In this case the fertilisation of the 

 ovum by a spermatozoon is followed by a great development, first of 

 the mucous membrane and later on of the muscular wall of the uterus. 

 The mucous membrane thickens, apparently in order to form a bed for 

 the developing fertilised ovum. With this growth of the uterus there 

 is a corresponding growth of the other parts of the genital tract, 

 e.g. the vagina. At the same time rapid changes take place in the 

 mammary glands. These changes may be studied experimentally in 

 the rabbit, in which animal gestation lasts only about twenty-nine 

 days. In a virgin rabbit of a year old it is difficult with the naked eye 

 to see any trace of the mammary gland in the tissue lying under the 

 nipples. Each gland is limited to an area not more than 1 cm. broad, 

 and consists entirely of ducts lined with a single layer of flattened 

 epithelial cells. With the occurrence of conception a marked change 

 takes place. Four or five days after fertilisation, when it is still 

 impossible with the naked eye to discover any embryos in the swollen 

 uterine horns, on reflecting the skin from the abdomen each mammary 

 gland appears as a circular pink area, about 3 cm. in diameter. On 

 section the gland consists of ducts which are in an active state of 

 proliferation, their epithelial lining being two or three cells thick and 

 presenting numerous mitotic figures. By the ninth day the whole 

 abdomen is covered with a thin layer of glandular tissue ; by the 

 twenty-fifth day this tissue is J cm. in thickness and consists for the 

 greater part of secreting alveoli, lined with cells containing numerous 

 fat -globules. At full term the alveoli contain ready-formed milk. 



This hypertrophy of the mammary glands occurs during pregnancy 

 after complete division of all possible nervous paths between the 

 glands of the ovaries or uterus. In the guinea-pig a mammary gland 

 has been actually transplanted to another part of the body, thus 



