REPRODUCTION. 415 



of lobules embedded in fat and areolar tissue. Each lobule is composed 

 of alveoli lined by columnar epithelium, resting on a basement 

 membrane. The ducts open on to the nipple, and are lined by cubical 

 epithelium ; their walls are said to contain unstriated muscular fibres. 

 During the secretion of milk the superficial part of the cells, which 

 contains fat globules and secretory granules, disintegrates and is cast 

 off to form part of the secretion ; during the periods of rest the cell 

 substance is re-formed. 



The development of the mammary gland during pregnancy is 

 brought about by the influence of hormones derived from the re- 

 productive organs. In the earliest stage of pregnancy a hormone 

 appears to be formed in the corpus luteum, and if a Graafian follicle 

 is artificially ruptured in an animal the mammary glands develop for 

 a short time apart from pregnancy. Later, another hormone is possibly 

 formed by the foetus itself, since it has been shown that injection of 

 an extract of foetal tissue into a virgin rabbit leads to growth of the 

 mammary glands. That their development is due to chemical and 

 not to nervous influence is further indicated by the fact that, even after 

 the severance of all nerves to the mammary gland, it undergoes normal 

 development during pregnancy. 



The Secretion of Milk. Very little is known as to the mechanism 

 by which the secretion of milk is brought about, although it can un- 

 doubtedly be influenced through the nervous system. As already 

 mentioned, the injection of extracts of the hypophysis increases the 

 secretion of milk, but it is not known whether the hypophysis normally 

 plays any part in the process. 



SUBSIDIARY FUNCTIONS OF THE REPRODUCTIVE ORGANS. 



The reproductive glands not only form the essential reproductive 

 elements, namely, spermatozoa and ova, but influence very markedly 

 the growth and development of the rest of the organism. 



In the male the onset of puberty, i.e. the formation of spermatozoa 

 in the testis, is associated with the development of secondary sexual 

 characteristics, such as changes in the larynx, deepening of the voice, 

 and the growth of hair on the face and pubes. If the testes are 

 removed before puberty these characters do not develop, and the body 

 remains infantile. After puberty castration leads to atrophy of the 

 accessory genital organs. In the lower animals castration also prevents 

 the appearance of secondary sexual characteristics, such as the antlers 

 of the stag, or the comb of the cock. 



In the female extirpation of the ovaries preve-nts the occurrence 



