218 MONTGOMERY A STUDY OF THE CHROMOSOMES 



ternal and paternal cells or stage of fertilization ; upon this follow a number of ovogonic 

 or spermatogonic generations, the exact number of which has not yet been determined for 

 any rnetazoon ; the last generation of the ovogonia or spermatogonia give rise to ovocytes 

 or spermatocytes of the first order, and these are characterized by the synapsis stage and 

 growth period when the reduction in the number of chromosomes is effected, the synapsis 

 stage being evidently coincident in all forms with the commencement of the growth 

 period ; and finally occur two maturation divisions which result in the formation of ovo- 

 tids or spermatids. The spermatids undergo an elaborate metamorphosis to become sper- 

 matozoa ; but since such a metamorphosis is not found in the ovotids, we may disregard 

 this stage, which evidently is far less conservative than the others ; from the comparative 

 standpoint the metamorphosis of the spermatozoon is of much less morphological signifi- 

 cance than the preceding stages of spermatogenesis, and would appear from the recent 

 investigations to be far more variable. 



Thus each germinal cycle shows the following well-marked stages: conjugation or 

 fertilization, a stage of a number of ovogonic or spermatogonic generations, the synapsis 

 stage coincident with the growth period, and the stage of the two maturation divisions. 

 Each such cycle is succeeded by a similar one, and so on indefinitely for an indefinite 

 number of cycles. Now it is unthinkable that a cycle should be without a beginning ; it 

 must have been gradually evolved, and some particular stage in it must have been the 

 starting point. What was this first stage? An answer is necessary before we can enter 

 into the discussion of the meaning of the synapsis stage. 



It appears to me most probable that the stage of conjugation of the germ cells must 

 be considered the starting point. For from the studies of R. Hertwig and Maupas 

 on Infusoria, it appears probable that conjugation or fertilization is essentially a pro- 

 cess of rejuvenation : cells may divide and reproduce for a number of generations 

 asexually, but there comes a period when the cellular vitality diminishes, so that no further 

 reproduction is possible except after rejuvenation afforded by conjugation with another 

 cell. When thus rejuvenated by admixture of substances from the other conjoint, the 

 cell starts upon a new period of generation the period of conjugation thus being the 

 commencement of a cycle. As we shall see, the synapsis stage is really a delayed part of 

 the process of conjugation, and the growth period is induced by the synapsis of the chro- 

 mosomes. Having determined the starting point of the germinal cycle, we may now con- 

 sider the meaning of the synapsis stage. 



(b) The phylogeny of chromosomes and the significance of the synapsis stage. 



In the considerations that follow I assume that through the germinal cycle the chro- 

 mosomes preserve their individuality from generation to generation i. e., that a particular 



