

THE STRUCTURE OF CELLS 43 



the lower plants and in the protozoa when more fully understood 

 will certainly shed light on the obscure phenomena exhibited by 

 the higher forms, and may ultimately give the clue for correctly 

 appreciating the general significance of the processes involved. It 

 has already been remarked that the exact point in the life-history 

 at which these remarkable divisions periodically recur is not identical 

 for all organisms, whilst the universality of the process indicates 

 clearly its great and fundamental importance. It has been urged 

 by some that the chromosomes, which are by those writers postu- 

 lated as permanent structures, become distributed between the 

 daughter cells in such a manner that only half of the original 

 number persist in each sexual cell. And in this way room is made 

 for the new ones imported in either of the two conjugating gametes. 

 Others again, like Oscar Hertwig, regard the quantitative reducing 

 of the chromatin (as opposed to that of the chromosomes) as the fact 

 of prime significance. In many of the lower forms, and notably in 

 the coccidia, the evidence tells strongly for this view ; for in them 

 it is the definite fact that a large part of the mother nucleus is left 

 unused when the gametes or gametocytes are produced, and thus 

 there is a quantitative reduction of a very pronounced character. 

 Again, in the same organisms the multiple division of the nucleus, 

 taken together with the amitotic division of the nucleus of the 

 zygote at segmentation, seem to tell equally against a mechanical 

 necessity for similarity in the chromatic strands. It is not easy to 

 believe in the permanent existence of specific chromosomes under 

 such circumstances ; but, on the other hand, there is no doubt that 

 if the different chromatic granules do really represent slightly 

 different structural characters, a qualitative reduction much in 

 the sense assumed by him may actually take place. For there 

 can scarcely exist any doubt but that, as the result of these pro- 

 cesses, the surviving parts of the mother nucleus do not represent 

 (especially after a multiple division) exact images of the original 

 nucleus from which they have sprung. But it would also seem to 

 be clear that whilst both a quantitative and a qualitative reduction 

 have taken place, these can hardly be regarded as direct means of 

 ensuring that an unvarying proportion of the original chromatin 

 shall be distributed amongst the daughter nuclei. Whether the 

 constant proportions observed in the higher forms is to be explained 

 as the result of a more definite constancy in the chromosomes, 

 together with the continuous existence of these structures in the 

 hypothetically more specialised nuclei of the higher animals and 

 plants, is a matter upon which it is as yet impossible to make any 

 positive statement. It may, however, be confidently asserted, having 

 regard to the extraordinary diversity which prevails in the details 

 of nuclear transformations in these lower forms, many of which will 

 be found described in the present volume, that amongst them, 



