224 MONTGOMERY A STUDY OP THE CHROMOSOMES 



Very evidently this great increase in volume is effected by that rejuvenescence of the chro- 

 mosomes secured by their conjugation. For the chromosomes are centres of metabolic 

 activity, by conjugation of paternal with maternal chromosomes in the synapsis stage 

 their metabolic functions are rejuvenated, and this rejuvenation finds its expression in the 

 great changes of the growth period. So this explanation of the. synapsis stage would seem 

 to be in accord with all the facts known at present. 



It is quite possible that at an earlier period in the phylogeny, the conjugation of the 

 chromosomes may have taken place at the time of the conjugation of the germ cells, and 

 not have been separated from that stage by a number of generations as in the modern 

 Metazoa. But the determination of the original time of occurrence of the conjugation of 

 the chromosomes is highly speculative, and so will not be entered upon here. 



It is generally stated (e.g. Von Rath, 1893 ; Riickert, 1894) that the bivalent chro- 

 mosomes of the spermatocytes and oyocytes of the first order are produced " by the spirem 

 segmenting into only half the normal number of chromosomes." This is not a correct 

 statement, since in the prophases of the first maturation mitosis there is, as I have shown 

 in my paper on Pcripatus, no stage of a continuous chromatin spirem. Further, this 

 general statement is not at all explanatory of the formation of bivalent chromosomes, for 

 it does not express any reason why the chromosomes should be joined into pairs. 



It is to Moore (1895) that we owe the first clear characterization and estimation of 

 the synapsis stage ; he divided the germinal cycle into the " first period " (conjugation of 

 germ cells, spermatogonic and ovogonic divisions), the "synaptic phases" (coincident with 

 the growth period), and the " second period " (maturation divisions). It will be seen that 

 my own classification of the stages is somewhat more detailed than Moore's, though it is 

 not necessarily any better. The important characteristic of the synapsis stage is, of course, 

 the union of chromosomes into bivalent pairs; the exact details of this process, which 

 appear to differ in different groups, are of secondary significance. 



(c) The significance of the maturation divisions. 



The two maturation divisions of the Metazoa represent the terminal stages of the 

 germinal cycle. 



In the Copepods (Hacker, 1895; Eiickert, 1894), the Isopods (Oniscus, in a just 

 finished paper by my student, Miss Louise Nichols), in the Insects (Von Rath, 1892 ;' 

 Henking, 1890; Montgomery, Paulmier, 1899 ; McClung, 1900), and in Peripatus (Mont- 

 gomery, 1901) there are well demonstrated cases that one of the maturation divisions is a 

 reduction division (pseudoreditclion, Riickert, 1894) in that it accomplishes a separation 

 of entire univalent chromosomes from one another. Such a reduction division, a trans- 

 verse splitting of the chromosomes, is not known for any other generation of the germinal 

 cycle, nor for any somatic generation. 



