416 ESSENTIALS OF PHYSIOLOGY. 



of menstruation and the development of the mammary gland, which 

 normally take place at puberty ; their removal after puberty brings 

 about the cessation of menstruation. 



The normal changes accompanying sexual development appear to 

 depend for their occurrence upon an internal secretion formed by the 

 testes or ovaries, and they furnish a striking illustration of the chemical 

 interrelation between the different parts of the body. The hormones 

 concerned in the growth of secondary sexual characteristics have not 

 been isolated, and the exact site of their formation is doubtful ; some 

 observers believe that they are formed by the interstitial cells of 

 the testis and ovary, since ligature of the vas deferens, while causing 

 atrophy of the seminiferous tubules, does not affect the interstitial cells 

 or the development of secondary sexual characteristics. 



HEREDITY. 



When reproduction takes place the offspring bear a general resem- 

 blance to the parents, though differing in detail from both of them. 

 The transmission of the qualities of the parent is carried out solely by 

 the germ plasm, and the hereditary qualities are present in the nuclei 

 of the spermatozoon and ovum respectively. The spermatozoon, how- 

 ever, appears to have a double function ; in addition to containing 

 potentially the characters of the male parent, it acts as a chemical or 

 physical stimulus to segmentation and division of the ovum. This 

 latter function can, to some extent, be replaced in certain invertebrates 

 {e.g. sea urchins) by producing physical changes in the environment of 

 the ovum ; and under such conditions unfertilised ova have been made 

 to develop partially or completely into normal larvae. 



The formation of polar bodies, and the analogous process taking 

 place in the formation of spermatids prior to fertilisation, involves 

 the loss of part of the nuclear substance, and therefore, probably, of 

 part of the parental qualities transmitted to the offspring. On this 

 account the characters of the offspring show variations from those of 

 the parents. In many cases these variations have proved advantageous 

 in the struggle for existence by adapting the animal more closely to its 

 environment ; and the survival of those most fitted for their surroundings 

 has led to the evolution of the higher forms of life. 



An important problem in heredity is whether the characters of the 

 offspring represent a mean between those of the parents, or whether a 

 parental character can be transmitted completely or not at all. There is 

 evidence that in many cases the latter is true, and the conditions under 

 which this transmission occurs are known as Mendel's law. When tall 



