<>K II 1 10 (iKRM CKLLS OF METAZOA. 



227 



taken as the criterion of the heterotypic mitosis. This signification would be different 

 from that originally defined by Flemming (1887), but it would certainly be a step toward 

 greater clearness to use " heterotypic " division in the place of I lie promiscuously used 

 "reduction " division. 



Now that we have seen that a reduction division occurs in the Copepoda, Insecta, Onis- 

 cus and Peripatus, and that the heterotypic division of the Vertebrata may be interpreted 

 as a reduction division also, we have to try to explain why such a reduction division occurs. 

 In the synapsis stage there is a conjugation of paternal with maternal chromosomes for the 

 purpose of rejuvenation of the chromosomes as metabolic centres, and this rejuvenation is 

 exemplified in the great metabolic activity of the growth period. Now, R. Hertwig and 

 Man pas have shown for the Infusoria that the two conjoints remain for only a certain 

 period in apposition, and that when the interchange of nuclei necessary for rejuvenation 

 has been accomplished the conjoints separate. Of course it is not a true analogy to com- 

 pare conjugating Infusoria (i. c, whole cells) with conjugating chromosomes (*. e., por- 

 tions of cells). But still it is very probable that two chromosomes unite temporarily for 

 the same reason that two Infusoria do, that is, for an interchange of substances; and 

 when the chromosomes have accomplished this interchange there would no longer be 

 any necessity for continued apposition, so they tend to separate from one another. It is the 

 reduction division in Metazoa which accomplishes the complete separation, though it may 

 commence in the prophases of this division. It is conceivable that the conjugated chro- 

 mosomes might, separate as they had come together, without the intervention of a mitosis. 

 But in the Metazoa, so far as we know, they become separated only by the agency of a 

 mitosis, and that is the reduction, mitosis. At the beginning of the germinal cycle there 

 is a repulsion of paternal and maternal chromosomes for one another, during the synapsis 

 a strong attraction, and at the end of the germinal cycle a, repulsion again, but not a re- 

 pulsion so strong as to distribute the chromosomes into two groups in the spermatid and 

 OVOtid. Only by a reduction division can paternal and maternal chromosomes become 

 wholly separated, for only then do the interchromosomal linin fibres (persisting portions 

 of the linin spirem : compare my paper on Peripatus) become broken. 



The question is complicated because another maturation division occurs, an equa- 

 tional division : in the Insecta, Oniscus, Peripatus and the Vertebrates the reduction 

 division precedes the equational, in the Copepoda (according to Ruckert, 1804) the re- 

 verse is the case. It is not difficult to explain why an equation division should occur at 

 this time, for the cell has increased in volume very greatly during the growth period, and 

 great increase in volume (increase beyond the individual mass) would appear to be a main 

 factor in inducing cell division. With this increase in volume of the cell the chromo- 

 somes also increase in volume (though by no means in a direct ratio), and each univalent 



