348 cook 



ences are preserved separately, like alternative sexual differ- 

 ences. Instead, therefore, of considering that Mendel's Laws 

 explain sexuality, it seems more reasonable to assimilate the 

 Mendelian phenomena with those of normal alternative descent 

 as shown generally in sex-inheritance. 



If the principle of alternative or polar heredity applies to 

 Mendelism, the earlier explanations by the special character- 

 units, segregated in different germ-cells, will be superfluous. 

 The phenomena would still be abnormal, as are the conditions 

 under which they appear, but they would no longer need to be 

 associated with the phenomena of incompatability of chromatin, 

 described by Guyer in sterile hybrids between diverse species. 



" When germ-cells are to be matured, before the real reduc- 

 tion, there is in most forms a so-called false reduction, in which 

 the chromosomes fuse in pairs so that there appears to be only 

 half the normal number present, though in reality each is 

 double (bivalent) and equivalent to two of the simple (univalent) 

 type. The doubling of chromosomes which normally occurs 

 at such times is frequently incomplete, or lacking, in hybrids. 

 This is especially true if the hybrids are from widely separated 

 species. Instead of a normal spindle bearing the usual number 

 of bivalent chromosomes, multipolar spindles, or two separate 

 spindles may appear, thus apparently permitting the two kinds 

 of parental chromatin to remain apart. In the most extreme 

 cases a complete separation may occur subsequently, the entire 

 chromatin of one parent occupying one cell, that of the other a 

 different cell. Such visible separations, however, only occur 

 extensively in sterile hybrids from markedly different parent 

 species. Fertile hybrids from closely related forms, for the 

 most part, display spindles normal in appearance. . . . 



" In the case of these milder fertile crosses, then, where rever- 

 sions follow the Mendelian law, the germinal incompatibilities 

 must be narrowed down to the qualities themselves rather than 

 confined to the respective germ-plasms as a whole. These 

 qualities must separate and each take up its abode in a different 

 germ-cell irrespective of whether the other qualities of that par- 

 ticular germ-cell are of a different parentage or not. The 

 cases in which the entire plasmas are segregated are then prob- 



