THE STRUCTURE OF CELLS 29 



it may happen that the subsequent events become much obscured, 

 the filamentous arrangement of the linin and chromatin ceasing to 

 be distinctly recognisable. 



Up to this period there is, on the whole, a general agreement 

 as to the nature and sequence of events, but the subsequent 

 changes have been very diversely described and interpreted in the 

 case of different organisms. 



In the most favourable cases the parallel arrangements of the 

 chromatin granules and of the split linin thread can be followed for 

 some time during the shortening arid thickening of the filaments. 

 Brauer has described, for the spermatocytes of Ascaris, a second 

 longitudinal fission in each chromatic filament, resulting in the 

 production of four rows of chromatin granules from the single row 

 originally present in the primitive thread. A similar event has 

 been stated by some to occur in the corresponding division in the 

 pollen-mother-cell of a lily. In the majority of instances, however, 

 the chromatic linin is seen to contract and thicken, and all traces 

 of the fission may become unrecognisable. Finally, the chromatin 

 comes to be aggregated in definite parts of the band, the intervening 

 portions being occupied by colourless linin. There are often, also, 

 cross anastomoses of the same substance between neighbouring 

 strands. The chromatic areas in question mark the position of the 

 developing chromosomes (Fig. 15, A\ which gradually become 

 more definitely isolated from each other. And they are seen to 

 be present in half the number characteristic of all the preceding 

 nuclear divisions in the organism. Once more each chromatic band 

 exhibits a split l along the whole or greater part of its length, and 

 this marks the line along which, later on, the cleavage of the 

 chromosomes in this (heterotype) mitosis will be effected. In many 

 cases, as is especially well seen in amphibians (Fig. 15, B, (7), the 

 fission of the young chromosome is incomplete and the sides diverge, 

 thus causing the whole to assume the form of a closed ring. In 

 other instances, however, the fission is completed, and the two 

 halves, lying in close juxtaposition, may exhibit a complex series 

 of figures which demand much care in their elucidation. 2 



1 It is commonly assumed that this split represents the original longitudinal 

 fission of the linin filament. It is not, however, proved beyond doubt that this is 

 invariably the case. 



2 These appearances have, however, been differently explained by some investi- 

 gators ; thus some have seen in them evidence of an approximation of two entire 

 somatic chromosomes, hence when the apparent halves separate to give rise to the 

 chromatic part of the daughter uuclt-i, it would follow that what has really 

 occurred is the distribution of half the original entire somatic chromosomes to the 

 daughter nuclei. That is to say, the division might be regarded as qualitative as 

 well as merely as quantitative. And it will be evident on reflection that the same 

 result might be reached as the result of various analogous interpretations of the fore- 

 going processes, especially when the difficulties of investigation that occur during the 

 synaptic tangle are borne in mind 



