GENERAL EMBRYOLOGY 



139 



Conjugation of the Chromosomes. — This reduction in tlie number of 

 chromosomes is explained l)y the plausible supposition (and this is supported 

 by many observations) that the previously separated pjaternal and maternal 

 chromosomes, and chromosomes with the same significance, have united (con- 

 jugation of chromosomes), and accordingly the allelomorphs, the equivalent male 

 and female determinants unite (in our example the determinants for red and 

 white flowers). If in this a complete fusion occur then there would result an 

 inseparable compound of characters, but if it be but a juxtaposition, then in 

 the further divisions of the sex-cells, the so-called maturation divisions, botli 

 kinds of determinants, paternal and maternal, would be separated again. The 

 latter would occur in cases following ^lendel's law. The white and red determi- 

 nants, present but only juxtaposed in the male sex cells, would be separated again 

 in one of the two maturation divisions (reduction division) and would be dis- 

 tributed among the resulting cells so that two cells would contain only white, 

 the other two only red determinants (fig. loo). The same would occur in the 



Fig. ioo. — Scheme of the maturation divisions of the sex cells to explain the splitting 

 of hybrid characters. It is based on the idea that the splitting, the reduction of the 

 chromosomes, occurs in the first maturation division, while many regard the second 

 as the reduction division. I and II, the first, III and IV, the second maturation 

 di\'ision. C, centrosomes; rf', d'-, centrosomes derived from different parents, which 

 are distributed among separate sex-cells. 



female sex cell, where at the end of maturation resulting in four cells — three 

 polar globules and the ripe egg — two would contain only white, the other two 

 only red determinants. Thus in the maturation divisions the sex cells lose their 

 hybrid character, and we speak of 'purity of gametes.' Now if such material 

 be used, as must be the case in close fertilization of crosses, there are four com- 

 binations possible, which according to the law of probabilities (we know no 

 reason favoring any particular combination) would occur in equal numbers; 

 (a) one fourth red male with red female, producing a true-breeding, homozygote 

 with red flowers, since no other determinant is present; (6) one fourth white 

 male with white female, producing homozygotes with white flowers; (c) one 

 fourth red males with white females; and (d) one fourth white males wdth red 

 females. The last two fourths would be red-flowered like the first, but accord- 

 ing to the composition of their cells, they are heterozygotes like their parents, 

 and like them, can be 'split' according to the same law. 



Mendel's law, illustrated above by a single example of hybrid peas, has 

 led to rich results, partly by Mendel himself, partly by his successors (Bateson, 



