D 



v HOMOLOGOUS CHROMOSOMES 127 



As they condense for metaphase the homologues become more distinct 

 from one another but still remain very closely paired (Fig. 56, B, G, I). 

 In some strains they remain indistinguishably fused even at this stage 

 (Taylor, 1915 a). In anaphase they again come into close application or 

 fusion (Fig. 56, E, J). 



It is interesting to note that in Taylor's material (1915 a, 1917), 

 although, in general, fusion of the homologous chromosomes was so 

 intimate that in somatic mitoses there appear to be only three chromo- 

 somes present, yet in the early cleavage divisions of the egg the six 

 chromosomes are as well separated from one another as in other animals. 



The case of the Diptera, and especially of Culex, leads to the conclusion 

 that the fusion of the chromosomes in syndesis is only the climax of a 

 general mutual attraction between homologous chromosomes. 



Further indirect evidence 

 of the continuity of the chromo- 

 somes is furnished by those 

 animals in which the bivalents 

 of the meiotic phase appear in 

 various different shapes. In 

 these animals it is found that 

 the same shapes, and the same 

 number of each shape, reappear 

 in every meiotic nucleus (Figs. 

 57-65). D E 1 E- r 



The one difficulty in the way FIG. 57. 



Of the hvDOthesis Of the COn- Tfle different forms of bivalents found in the meiotic 



phase of the newt. (After Moore and Arnold, P.R.S., B 1906.) 



tinuitV Of the Chromosomes IS Two of each type ^ e found in all primary spermatocytes 



(El, 2, being alternative forms of the same type). 



the fact that in the great 



majority of cases they lose all visible signs of their identity in the 

 resting nucleus. This, however, is a piece of negative evidence which 

 cannot be allowed to outweigh the overwhelming indirect evidence from 

 their constancy in number, relative sizes, etc., which indicates that they 

 do actually maintain this continuity. Moreover, in many cases direct 

 evidence has been obtained that the chromosomes which enter into the 

 resting nucleus at telophase do not become diffused throughout the 

 whole nucleus and inextricably mingled up with one another, but retain 

 a definite localization in the nucleus though their boundaries may not 

 be visibly distinguishable. The classical piece of evidence on this 

 head is Boveri's work on the cleavage nuclei of Ascaris megalocephala, 

 which on account of the small number of its chromosomes is plainly 

 a favourable object for such investigation. 



Originally carried out on the eggs of Ascaris megalocephala bivalens 

 (Boveri, 1888), the work was repeated by Boveri in 1909 on the univalens 



<no 



