CHROMOSOME COMPLEX OF SPERMATOCYTES. 335 



have simple chromosomes of equal size, but when the decad 

 divides it would appear as though there were some heterogeneity 

 present, for in the anaphase one limb of the loop is longer than 

 the other (Fig. 13). This may be due to the formation of a 

 multiple chromosome partly from the accessory chromosome ; 

 otherwise it means that the tetrad is not constituted of homolo- 

 gous simple chromosomes. Aside from this there seems to be 

 nothing to contradict the view that the tetrads represent the union 

 of homologous paternal and maternal elements. 



4. Pliylogcny of the Chromosomes. 



By the determination of the fact that for the family Acrididae 

 the number of chromosomes is a constant we gain a new view- 

 point for the study of chromosome descent. So long as it was 

 thought that nearly related species were characterized by the pos- 

 session of different numbers of chromosomes it was impossible to 

 regard the individual chromosomes as very constant structures. 

 We have accordingly such theories as Paulmier's, later endorsed 

 and elaborated by Montgomery, by which certain chromosomes 

 smaller than the others and somewhat different in their behavior 

 in the mitoses are regarded as disappearing from the species and 

 carrying with them the loss of certain characters. 



On a priori grounds such an occurrence would seem to be 

 very improbable, for the difference between any two species is so 

 slight that the loss of any number of .the chromosomes would be 

 entirely disproportionate to the effect. As a matter of fact the 

 difference between two species or two genera consists not so 

 much in the presence or absence of certain characters, but rather 

 in the modification of those possessed in common. If therefore 

 we have a constant relation between a certain group of chromo- 

 somes and a certain series of body characters we would expect 

 differences between individuals to come about, not by the loss of 

 chromosomes but by the modification of their structure or rela- 

 tions. .It seems to me that the incomplete observations which I 

 am presenting in this paper strongly indicate that this is what 

 takes place. 



The Acrididae are a group of grasshoppers well marked off from 

 the other Orthopteran families by the possession of a series of 



