EVOLUTION, HEREDITY, EUGENICS 503 



Gametogenesis {gametes, spouse; and genesis, root or origin). — 

 The essential organs for reproduction are the ovaries and the testes. 

 In the ovary are formed the ova, or eggs, and in the testes are formed 

 microscopic elements called spermatozoa. Except in a few of the 

 lower animals, the development of an ovum into a new individual 

 is made possible only by its fertilization by a spermatozoon. (See 

 page 506.) 



Prior to the process of fertilization there are certain preparatory 

 changes which have an important bearing on the condition of the 

 mature eggs and sperms. During these preliminary changes, we 

 find a series of ?nitoses (see page 500) accompanied by a rather 

 striking method of reduction in the number of chromosomes such 

 that only one-half the original number is present in egg and sperm 

 when fertilization takes place. In the body cells and the early 

 germ cells, the chromosomes occur in duplicate series, one member 

 of each series being derived from the male and the other from the 

 female parent. The gametes (mature germ cells) each contain a 

 single {hafloid) group, while the fertilized egg (zygote) contains a 

 diploid group. 



Since the chromosomes are the only structures known to be 

 contributed equally by the two parents and since offspring inherit 

 equally from both parents, except in regard to sex and sex-linked 

 characters (see page 535), the chromosomes are believed by most 

 biologists to be the bearers of the hereditary determiners or genes. 

 (See page 538.) Correlated with this view is the belief that each 

 chromosome maintains its individuality through the various phases 

 of mitosis. Evidently, then, the number of chromosomes would be 

 doubled at each fertilization without some mechanism for keeping it 

 constant. 



Fogg (Jour. Morphol., 1930, vol. 50) has a new and striking con- 

 ception of chromatin diminution quite at variance with the texts 

 and monographs now extant. He holds that the diminution of 

 chromatin is not directly concerned with genetic variations, and 

 plays no primary essential part in differentiating the germ line from 

 the somatic. 



In Ephestia, Fogg shows that It is clearly non-genic. It is to be 

 considered rather as the casting out of residual material sooner or 

 later in the mitotic stages. 



Spermatogenesis (formation of spermatozoa). — The sperma- 

 togonia undergo a period of multiplication by mitotic divisions (see 



