34 THE STRUCTURE OF CELLS 



stages and divide at the equator, each half (monad) forming a 

 daughter chromosome. 



And an essentially similar condition obtains in at least many 

 other more obscure instances. Thus in Salamander and in Trades- 

 cantia, during the dyaster condition of the anaphase of the hetero- 

 type, each daughter chromosome is seen to be longitudinally divided 

 (Fig. 15, F). This almost certainly is the result of the reopening of 

 a split formed during the prophase of the heterotype, but which has 

 escaped recognition in many cases owing to the great difficulties- 

 which these earlier stages present in their investigation. And the 

 fission, thus obvious in the dyaster of the heterotype, provides the 

 daughter chromosomes for the next (homotype) mitosis. 



It is thus seen that these two mitoses, the heterotype and the 

 homotype, which in animals are the immediate forerunners of the 

 differentiation of the sexual cells, are clearly distinguishable from 

 the preceding ones in several important respects. 



1. The appearance of the chromosomes in the reduced number, 

 i.e. they are only half as numerous as in the rest of the nuclei. 



2. The long duration of the prophase, and the complex changes 

 and rearrangements, including, probably universally, a preparation 

 for the distribution of daughter chromosomes not only for this but 

 also for the succeeding division. 



3. The remarkable forms assumed by the chromosomes upon 

 the spindle. 



4. The very general extrusion of nucleolar substance, in rela- 

 tively large quantities, from the nucleus during the prophase. 

 Although probably of importance, it would be as yet premature to 

 speculate on the precise weight to be attached to this phenomenon, 

 but it is suggestive when considered in relation to the course of 

 events described for protozoa. 



The accompanying figure may serve to render clearer the exact 

 nature of the different views which have been held as to the nature 

 of the processes which are passed through in the reduction divisions. 

 The somatic cell (I. in the figure) is supposed to include two 

 chromosomes, and below are represented, diagrammatically, the 

 various alternative phases gone through during reduction, the 

 corresponding stages being shown in any four figures on the same 

 horizontal line. The series II. and III. represent the events which 

 may be passed through on the assumption of the permanence' of the 

 chromosomes, whilst the series IV. and V. correspond to those in 

 which such a permanence is denied. In the former case the two 

 original chromosomes, A, B, remain temporarily united, and their 

 two methods of possible . separation are respectively shown. In 

 IV. and V. no continuous identity is claimed for the chromosomes, 

 and the two original ones are replaced by a single new one (C). 



Considerable difference of opinion exists, then, as to the real 



