274 EVOLUTION AND ANIMAL LIFE 



a-b-c-d the individual chromosomes, and -> r > -> -. their splitting 



abed 



longitudinally in ordinary division. In the maturation of 

 the egg and in spermatogenesis, however, the thread segments 



. , , , ,., , ., ,. abab cdcd 



into ab, cd, and splits twice longitudinally into r-r - ; 



^^^^ ao ao ca cd 



the two tetrads of B in Fig. 155. The^B ^ion of chromatin 

 here is only a reduction in mass and Balitative one, in 

 Weismann's sense, as in the Crustacea ^^^Piects. In Ascaris 

 the actual reduction in number of chromosomes takes place 

 in the nucleus previous to the maturation divisions of the 

 ovocyte and spermatocyte ^pectively. In Cyclops the for- 

 mation of the tetrads is merely a pseudo-reduction, the actual 

 reduction taking place in the second division, which gives rise 

 to the mature egg on the one hand, or the spermatids, which 

 develop into the spermatozoa, on the other. 



One fundamental fact is clear in these divergent accounts. 

 The number of chromosomes is reduced in both sorts of the 

 germinal cells as a preliminary to their union. Whether there 

 is likewise a qualitative distribution of the chromatin elements 

 remains for future investigation to decide. From the facts of 

 ordinary cell division we have seen that the chromatin of 

 the nucleus is to be regarded as the bearer of hereditary qualities 

 in the cell. The phenomena of fertilization greatly increase 

 this probability. The offspring resembles both of its parents, 

 and the paternal tendencies can be conveyed in the minute 

 spermatozoan head alone, which is constituted almost entirely 

 of chromatin. The scrupulous exactitude with which, in both 

 germ cells, the chromosomes are reduced to one half the normal 

 number preparatory to the union of the pronuclei in fertiliza- 

 tion, and the distribution of the paternal and maternal chro- 

 matin equally to the resulting cells of cleavage, lend added 

 weight to the theory. 



The development of the fertilized germ cell into the com- 

 plete organism is discussed in the preceding chapter as also 

 is the significance of sex. This significance in the light of actual 

 processes of germ-cell formation, maturation, and fertilization 

 is seen to be very important in relation to the phenomenon 

 of variation, a phenomenon or fact which we have already 

 learned to recognize as the absolutely essential basis of all 

 organic evolution. 



