543 
in Pimis and Thuja. 
ance of the theory of the individuality of the chromosomes ; for, if the 
chromosomes are to be regarded as individuals, it is clear that reduction can 
take place only when one of the two tetrad or maturation divisions is 
transverse. 
The belief in a true reducing division is not new, but its general 
adoption as a true process of meiotic division is comparatively recent. 
Probably the greatest stimulus to research along this line was exerted by 
the re-discovery of the Mendelian principle of the segregation of characters 
in certain inbred hybrid forms. 
As long ago as 1884, Heuser asserted that there occurs in Tradescantia 
virginicct a transverse division of the chromosomes in the first mitosis, and 
that this mitosis effects, therefore, a qualitative division. Korschelt (’95) 
reported similar conditions in the annelid Ophryotrocha pnerilis , but neither 
of these investigators succeeded in convincing subsequent observers of the 
accuracy of his work, and it was passed unheeded. It was not until experi- 
mental investigators had re-established the Mendelian principle of character 
segregation in hybrids that botanists began with renewed energy the search 
for a reducing and a qualitative mitosis. 
Too much credit cannot be given to the plant breeders for the valuable 
contributions they have made in the form of statistical reports of the out- 
ward manifestations of hereditary characters in many species of plants. 
This work, while furnishing valuable criteria for the cytologists, has done 
more to dispel the idea that fecundation consists in the chemical union of 
the two fusing gamete-nuclei than could ever have been accomplished by 
the student of cytology alone. The students of experimental research on 
heredity have been driven to assume the material primordia of characters 
which the cytologists have been able to demonstrate as existing in the germ 
nuclei, and thus the two working from directly opposite sources have 
reached the same conclusion, a coincidence which must be regarded as of 
the greatest significance. There seems no longer any doubt that there 
exist in each gamete-nucleus certain material primordia which are in some 
way responsible in the determination of the characters of the resulting 
organism. That these primordia, or determinants, are definite chemical 
compounds seems quite probable, although nothing explicit can be offered 
along this line at the present time. All the experimental work of the past 
argues strongly against the idea that these primordia fuse at fecundation in 
such a way as to give rise to an entirely new hereditary substance. On 
the contrary investigation seems to bear out the view that the primordia are 
merely mixed in a mechanical way, but without chemical union. Whether 
these material primordia of characters be regarded as the chromosomes 
themselves, as has been generally the case, or as the smaller particles of 
which they are composed, as suggested by Weismann and variously modified 
by De Vries, Strasburger, Mottier, Farmer, and others, it seems quite 
