212 MONTGOMERY A STUDY OF THE CHROMOSOMES 



value it would be a signboard of degree of specialization within the group. From 

 such a point of view it might even be possible to construct a cellular classification which 

 would Iiavf great value in that it would employ truly conservative structures. The cen- 

 trosomes and central spindles have been considered phyletically in this way by Biitschli, 

 Lauterborn, Heidenhain, Calkins and others, but so far the chromosomes have not been 

 considered from such a standpoint, although they in many respects appear more conserv- 

 ative than centrosomes and achromatic spindle structures. 



Accordingly, though the present study has not given a solution to the problem of 

 the factors governing the number of chromosomes, except in showing that it must be 

 sought in the phenomena of metabolism, yet it would show that chromosomal number may 

 be employed as a criterion of relationship if it be used with caution and with due consid- 

 eration of a broad comparative treatment of other. structures. And the reason is because 

 the chromosomes seem to be highly conservative, their number constant for the species, 

 and because in a certain sense they -represent the most important vital structures. We 

 should not conclude that all forms with e. g. ten chromosomes should be ranked as closely 

 related ; all with e. g. twenty-four as composing another natural group. But within a 

 certain group which has been defined on a broadly comparative basis such a group as the 

 Hemiptcra hcteroptcra for instance, the chromosomal number would perhaps be a clue 

 to the relative degree of specialization of the species. 



(c) Significance of the uneven normal number. 



One of the most unexpected results of this investigation was the discovery that in 

 some species there is an uneven number of chromosomes in the spermatogonia, i. e., an 

 uneven normal number, whereas heretofore all observation and assumption has been that 

 the normal number is always an even one. Of the Hemiptera studied, four species show 

 an uneven number of chromosomes in the spermatogonia, namely, Alydus eurinus, Har- 

 mostes reflexulus, Protenor belfragei and (Edancala dorsalis ; in all of these the number 

 is eleven, and in the synapsis stage one chromosome remains univalent while the ten re- 

 maining combine to form five bivalent chromosomes. AVhat is the significance and origin 

 of this uneven number? 



Now, as far as our facts go, it seems that the number of the chromosomes is constant 

 for the species, and that the paternal and maternal germ cells of a species have the same 

 number ; this appears to be one of the points in the correspondence of the ovogenesis and 

 spermatogenesis first determined by Henking (1890) and O. Hertwig (1890). Whenever 

 there is this correspondence in number and valence, then in fertilization, when the 

 paternal chromosomes- are added to the maternal, an even number of chromosomes 

 should result, and if the chromosomes maintain their individuality through the succeed- 



