198 



MONTOOM I>:rY — A STUDY OP THE CSEOMOSOMES 



found after tlic action of most diverse fixatives. The dense and interlacing grouping of 

 the chromosomes that is so characteristic for the synapsis of Insects, Copepods, Ascaris 

 and some other forms, is, however, not found in all ; thus it does not appear to occur in 

 Salamandva (Moves, 1890). 



Moore (1895) first gave (lie name "synaptic phase "to that, stage in the growth 

 period of Elasmohranehs when the reduction in the number of the chromosomes lakes 

 place. Accordingly, the criterion of the synapsis stage is first of all the combination of 

 univalent chromosomes to form bivalent ones ; whether the chromosomes are then densely 

 grouped or not is o( secondary importance. A special chapter of the present [taper is 

 given to the broader significance of this stage. 



In all the Hemiptera examined, and also in Peripatw, I have found that bivalent 

 chromosomes are formed in the synapsis by a union, end to end, of every two univalent 

 chromosomes. All other writers on this stage, with the exception of Brauer, have been 

 unable to determine how the univalent chromosomes become united together; Brauer s 

 (1893 b) careful study of the growth period in the spermatogenesis of Ascaris rendered 

 it very probable that it is a union end to end, but his figures do not prove it absolutely. 

 Certain writers state (bat. the reduction of the number in chromosomes is effected by the 

 chromatin spirem segmenting into only half the normal number of chromosomes. This 

 is, however, an incorrect statement, inasmuch as the reduction in number is occasioned in 

 some cases (Hemiptera, Peripatus) before; the " spirem " stage of the first maturation 

 division, and inasmuch as in most cases, if not all, there is no continuous chromatin 

 spirem found at any time during the growth period and prophases of the first maturation 



mitosis. 



Accordingly in the Hemiptera the reduction in number of the chromosomes is 

 effected in the synapsis stage, a long while before the maturation divisions, by a union 

 cud to end of every two chromosomes. During the synapsis stage the chromosomes 

 become split longitudinally, as was first shown by Paulmier (1898, 1899) for A nam — 

 a process that I had overlooked in my former paper (1898). Each bivalent chromosome 

 is thus both transversely and longitudinally split before the maturation mitoses, the 

 transverse split represented by the band of linin joining the approximated ends of the 

 two univalent, chromosomes. 



At t he close of the growth period there is a well-defined rest stage in most Hemiptera, 

 when chromosomal boundaries are practically indistinguishable ; but in the Coreidce and 

 Redumidce there appears to be no such stage, and accordingly such a stage would appear 

 to have no broad significance!. 



In the early prophases of the maturation divisions the chromosomes are bivalent 

 but quadripartite, each one being transversely and longitudinally split; in the later pro- 



