352 The Bearing of Cytological Research on Heredity. 



chromosomes as the " bearers of heredity " we are employing a figure of 

 speech. They are such just to the extent that they are necessary to develop- 

 ment and heredity ; but how far this conclusion carries we are as yet 

 unable to say. Genetic experiment has already given some ground for the 

 conclusion that definite types of hereditary distribution may be immediately 

 dependent upon elements contained in the protoplasm. Eecent advances in 

 our knowledge of the " chondriosomes " or " plastosomes " provide this conclu- 

 sion with at least a possible cytological basis. 



Our conceptions of cell-organisation, like those of development and heredity, 

 are still in the making. The time has not yet come when we can safely 

 attempt to give them very definite outlines. It is our fortune to live in a day 

 when the business of observation and experiment leaves little time or 

 inclination for a priori speculations concerning the architecture of the germ- 

 plasm or of the cell. Nevertheless it is impossible not to be struck with 

 the fact that recent advances in cytology and genetics are in certain important 

 respects in line with theoretical views put forward nearly 30 years ago by 

 Roux, Weismann, and de Vries. These views were, it is true, almost purely 

 imaginative or logical constructions. Some of- them, especially as applied to 

 the mechanism of embryological development, have been experimentally 

 disproved, others are incapable of verification, and hence have fallen into 

 disrepute. We have become chary of theories which assume all parts of 

 the cell to be built up of ultimate, self-propagating, vital units, such as 

 " gemmules," " pangens," or " biophores." The working hypothesis that has 

 here been considered must not be identified with those far-reaching specula- 

 tions ; it is at once more limited in scope and more flexible in form. And 

 yet, as far as the cell-nucleus is concerned, those visions of a bygone 

 speculative era are now beginning to seem more real than would have 

 been thought possible by some of us ten or even five years ago. We read in 

 the latest productions of cytology and genetics of the division and genetic 

 continuity of factors or gens, of their linear alignment in the chromatin 

 threads, of their conjugation and disjunction, of their linkage or independent 

 distribution, in heredity. We find such conceptions no longer treated as 

 belonging to an age of cytological romance, but employed every day in the 

 most matter-of-fact way as practical instruments of laboratory experiment, 

 analysis, and prediction. We are bound to no speculative systems or extrava- 

 gances of an earlier day if we recognise in this, let the outcome be what it 

 may, a triumph for the men who first endeavoured to bring cytology and the 

 experimental study of heredity into organic relation. 



