Experimental Hybridizing 75 
cells the three kinds of individuals of the second generation. A 
white spermatozoén meeting a white egg, as one may say for 
brevity, produces a “pure” recessive individual; a gray sperma- 
tozodn meeting a gray egg gives a “‘pure”’ dominant individual ; 
but when a white spermatozobn meets a gray egg, the off- 
spring will be mixed, a heterozygote, and like the first 
hybrid (F;). 
If this hypothesis is true, we ought to be able to determine 
which groups of characters — where several are involved — 
are contained in a given chromosome, for the number of chro- 
mosomes is often small, and therefore the actual number of 
combinations is limited. All characters contained in the same 
chromosome should “‘Mendelize’’ together. 
If we consider two characters, the principles stated above 
will apply. Let us take the case of the round and yellow, 
angular and green. peas. The round form, A, may become 
combined with the yellow, B, or with the green color, 6, giving 
AB or Ab; the angular form a with B or 8, giving aB or ab. 
To work this out we must suppose the color to be contained 
in a different chromosome from the form, for otherwise they 
could not shift over. When the chromosomes unite in synapsis 
the A-bearing chromosome can only unite with @, and B can only 
. may be turned either way 
unite with 6. Hence since x and 
on the spindle, A may pass into a cell with B or 0, and also a 
into a cell with B or 8. 
Simple and logical as Mendel’s assumption appears to be on 
the hypothesis of each character being contained in only one of 
the chromosomes, yet I do not think it can be accepted in this 
- form because the primary assumption that each character is 
contained in a single chromosome and not in others is highly 
arbitrary, and also because there are certain actual results that 
are difficult to explain on the assumption of “pure” germ-cells. 
In fact, it has not been shown beyond question that the chro- 
mosomes are the bearers of the hereditary qualities. The evi- 
dence that is generally supposed to establish the assumption of 
