CHROMOSOME GROUP IN BRACHYSTOLA MAGNA. 39 



If, as the facts in Bracliystola so strongly suggest, the chromo- 

 somes are persistent individuals in the sense that each bears a 

 genetic relation to one only of the previous generation, the proba- 

 bility must be accepted that each represents the same qualities as 

 its parent element. A given relative size may therefore be taken 

 as characteristic of the physical basis of a certain definite set of 

 qualities. But each element of the chromosome series of the 

 spermatozoon has a morphological counterpart in that of the 

 mature egg and from this it follows that the two cover the same 

 field in development. When the two copulate, therefore, in syn- 

 apsis l the entire chromatin basis of a certain set of qualities in- 

 herited from the two parents is localized for the first and only 

 time in a single continuous chromatin mass ; and when in the 

 second spermatocyte division, the two parts are again separated, 

 one goes entire to each pole contributing to the daughter-cells 

 the corresponding group of qualities from the paternal or the 

 maternal stock as the case may be. 



There is, therefore, in Bracliystola no qualitative division of 

 chromosomes but only a separation of the two members of a pair 

 which, while coexisting in a single nucleus, may be regarded as 

 jointly controlling certain restricted portions of the development 

 of the individual. By the light of this conception we are enabled 

 to see an explanation of that hitherto problematical process, 

 synapsis, in the provision which it makes that the two chromo- 

 somes representing the same specific characters shall in no case 

 enter the nucleus of a single spermatid or mature egg. 



I may finally call attention to the probability that the associa- 

 tion of paternal and maternal chromosomes in pairs and their sub- 

 sequent separation during the reducing division as indicated above 

 may constitute the physical basis of the Mendelian law of hered- 

 ity. To this subject I hope soon to return in another place. 



I take pleasure in expressing here my gratitude to Prof. E. B. 

 Wilson for much valuable advice and assistance in the work 

 upon BracJiy^tola and in the preparation of the present paper. 



ZOOLOGICAL LABORATORY, COLUMBIA UNIVERSITY, 

 October 17, 1902. 



1 The suggestion that maternal chromosomes unite with paternal ones in synapsis 

 was first made by Montgomery ( 1901, I.). 



