INHERITANCE 



299 



.^P. 







« V Ml, 



Fig. 178. Diagram of the sepa- 

 rate maintenance of paternal 

 and maternal chromosomes 

 as seen in certain hybrids. 

 5^, sperm; o, egg; a, the form 

 of the chromosomes of the 

 sperm; b, the form of the 

 chromosomes of the egg; w, 

 fertilization about to take 

 place; x, the nucleus in its 

 succeeding resting stage; y, 

 the reappearance and group- 

 ing of the two sorts of chrom- 

 r)mes at a subsequent divi- 

 sion; z, division of the cyto- 

 plasm. The condition in each 

 nucleus is diagrammatically 

 indicated by the circles below. 



Fertilization and maturation. — 



The existence of sperm cells has 

 been known ever since the great 

 pioneer Dutch naturalist Leeuwen- 

 hoek and his pupils with home 

 made lenses found them in the 

 seminal fluid of animals, but they 

 were long regarded as' 'wild animal- 

 cules." In 1875, Oscar Hertwig 

 established the fact that fertiliza- 

 tion consists in the union of one 

 egg and one sperm only, showing 

 that in sexual reproduction each 

 parent contributes one cell of its 

 own body to the formation of the 

 young. Then it became evident 

 that the sexes play an equal, al- 

 though not necessarily an identical 

 role, in hereditary transmission. 

 This conclusion was strongly re- 

 enforced by the important dis- 

 covery of Van Beneden (1883), 

 that germ cells contain but half 

 the number of chromosomes that 

 is normal to the body cells of their 

 own species. It became evident, 

 therefore, that reduction and fer- 

 tilization are complemental pro- 

 cesses, the one leaving each sex 

 cell with but a half stock of chro- 

 mosomes, ■'"he other restoring: to 

 the fertilized egg cell the normal 

 number. At the same time, the 

 sperm introduces new elements 

 into the lineage of the egg cell; 



