250 WALTER S. SUTTON. 



all extremes of chromosome combination are provided for within 

 the limits of each sex. 



POSTSCRIPT. 



The interesting and important communication of Guyer l on 

 " Hybridism and the Germ-Cell'' is received too late for con- 

 sideration in the body of this paper. This investigator also has 

 applied conclusions from cytological data to the explanation of 

 certain phenomena of heredity, and his comparative observations 

 on the spermatogenesis of fertile and infertile hybrids are an im- 

 portant contribution to the cytological study of the subject. The 

 conclusions drawn are of great interest but, I think, in some cases, 

 open to criticism. In assuming that there is a "segregation of 

 maternal and paternal chromosomes into separate cells, which 

 may be considered ' pure ' germ-cells containing qualities of only 

 one species" (p. 19), he repeats the error of Cannon which has 

 already been dealt with in the early part of this paper. No 

 mention is made in the paper of Mendel's law but in considering 

 the inbred pigeon hybrids from which his material was obtained, 

 the author expresses his familiarity with manifestations of the 

 Mendelian principle by the statement that " in the third gener- 

 ation there is generally a return to the original colors of the 

 grandparents." In cases which seem to resemble one grand- 

 parent in all particulars it is clear that the conception of pure 

 germ-cells may be strictly applied, but the author was familiar 

 with cases of inbred hybrids which plainly show mixtures. 

 These he is inclined to explain in two ways as follows : ( i ) 

 " Union of two cells representing each of the two original species 

 would yield an offspring of the mixed type." (2) " Besides 

 through the mixing just indicated, variability may be due also in 

 some cases to the not infrequent inequalities in the division of 

 individual chromosomes, through which varying proportions of 

 the chromatin of each species may appear in certain of the 

 mature germ-cells" (p. 20). 



The first of these explanations would accord with the result of 

 Mendelian experiment but for the fact that it is erroneously ap- 

 plied (and without cytological grounds) to all the characters or 

 chromosomes instead of to individuals. As for the second 



1 Guyer, M. F., " Hybridism and the Germ-Cell," Bulletin of the University of 

 Cincinnati, No. 21, 1902. 



