i64 SCIENCE PROGRESS. 



transversely, what becomes of the elaborate theories which 

 have been founded upon the assumption of the universality 

 of one or other of these two processes ? The believed equiv- 

 alence secured by a longitudinal fission was supposed to 

 indicate a regular arrangement of the substance of the 

 chromosome of such a nature that, although the composi- 

 tion might vary from one end to the other, the composition 

 of any thin transverse plane could be assumed to be homo- 

 geneous. And the alternating discs of strongly and slightly 

 stainable matter described by Balbani ^ in Chirononnts, and 

 seen by Strasburger in many plants, were thought to be the 

 optical expression of this serial arrangement. 



Since Weismann's theory has dominated a great part of 

 cytological investigation, it has been assumed that this struc- 

 ture corresponds to a definite arrangement of the bearers of 

 hereditary qualities. The chromosome is conceived of as a 

 vastly complex structure, made up of small individual units 

 which are grouped in a particular manner. But how does such 

 a theory fit in with those facts concerning reduction which 

 are now before us ? It seems that if the chromosomes do 

 really consist of a number of discrete bearers of hereditary 

 qualities, of particles which must themselves be very stable, 

 since the qualities they are assumed to bear are, as a matter 

 of fact, remarkably constant ; — it seems that the only way of 

 making room for the fresh qualities introduced at the act 

 of fertilisation is first to get rid of half the number of entire 

 chromosomes from the sexual nuclei. This may be readily 

 imagined in the case of Copepods, but how can we admit it 

 in the other cases I have quoted } But in order to try to 

 reconcile the doctrine of determinants and so forth, with the 

 facts of reduction as exhibited in plants and, at any rate some, 

 animals, it has been suggested that in the resting period before 

 the first division the chromosomes really fuse end to end 

 just as they are said to do in Cyclops, but that they never 

 break asunder again. But this explanation lands us in all 

 sorts of difficulties when we try to work it out. The de- 

 terminants, biophores and so on, are, ex hypothesis stable 



^ " Sur la structure du noyau des cellules saliraires chez les larves de 

 Chironomus," Zool. Ans., 1S82. 



