144 CYTOLOGY 



CHAP. 



the " type " chromosomes. It is significant that he found no fragmenta- " 

 tion in the germinal tissues. ^ 



It is plain that variation in chromosome number by linkage or 

 fragmentation is not by any means incompatible with the general thesis J 

 laid down on p. 123. The primary object of the arrangement of the 

 units in Unear series, which is to allow of their accurate distribution to 

 the daughter nuclei after fission in mitosis, is not in the least prejudiced 

 thereby. Syndesis of homologous chromosomes, or rather of the units 

 composing them, would, however, certainly be complicated if irregular 

 fragmentation were to occur in the germ track. It is precisely here, how- 

 ever, as the examples above quoted suffice to show, that the chromosome 

 number is particularly constant. Only a few cases of fragmentation or 

 linkage within the germ tracks of individuals have been described, and 

 these are mostly of a simple and orderly nature. The evolution of a 

 species with a chromosome number differing from that of the parent 

 species must indeed have been accompanied by a rearrangement of 

 chromatin units, or by a fragmentation or linkage of existing chromo- 

 somes. This case, however, goes far to furnish proof of the necessity for 

 the constancy of the linear arrangement, since there is strong reason to 

 beheve that interspecific sterility arises through incompatibility of the 

 chromosomes of the two incipient species in syndesis (Chapter VI.). 



The remaining two causes of variation in chromosome number do 

 not call into question the continuity of the chromosomes, since they 

 concern the multiplication or disappearance of whole chromosomes. 



(2) Variation in Chromosome Number due to Irregularities of Mitosis 



This is an abnormahty which, if leading to an extensive loss of 

 chromosomes, must lead eventually to the death of the cell in which, 

 it has occurred. Since each diploid nucleus contains a double set of 

 chromosomes, and since one set contains all the units required for a 

 perfect organism, it follows that one member of each homologous pair 

 might be lost without much disturbance, but if both members of a pair 

 disappeared serious consequences must result. 



Though mitotic irregularities usually lead to a loss of chromosomes 

 owing to one or more of these faiUng to get included in the daughter 

 nuclei, they sometimes have the effect of increasing the number. Probably 

 the earhest of these cases to be described was in Ascaris megalocephala 

 hivalens (Fig. 70) . Boveri (for summary see 1904) found that sometimes 

 the spindle of the first meiotic division in the e^^ is placed tangentially 



1 This perhaps is an indication of a differentiation between soma and gonad as displayed 

 in the germ tracks of so many animals. 



