76 Experimental Zoology 
the hereditary nature of the chromosomes is that the spermato- 
zoon brings into the egg only the chromosomes of the male 
germ-cells. While it is undoubtedly true that the largest part 
of the sperm-head is made up of chromatin material derived 
from the nucleus, it is also true that the protoplasm of the origi- 
nal germ-cell is not lost. It is partly used up in the formation 
of the tail of the spermatozo6én, but also forms a condensed 
layer around the sperm-head. There is every reason to assume 
that the latter may become incorporated as a part of the cyto- 
plasm of the egg. There is also another serious objection to the 
explanation of the purity of the germ-cells given above, for, even 
if the chromosomes be the bearers of the hereditary qualities of 
the egg and sperm, it does not follow that each unit character 
would be contained in only one chromosome. If it be assumed 
that each chromosome carries all the hereditary qualities, it is im- 
possible to account for the purity of the germ-cells on this as- 
sumption. For instance, if we assume that each of the chro- 
mosomes contains all of the hereditary characters, the germ-cells 
of the hybrid, A(B), will contain, before synapsis, half of their 
chromosomes bearing the character A, and half bearing the 
character B. If these unite at the synapsis in pairs, and then 
come to lie, haphazard, on the spindle, some turned one way, 
some the other, the resulting germ-cells will contain all 
mixtures of A and B, and hence be impure. If we give up 
the idea of “purity” and assume that the relative number of 
A or of B chromosomes determine the character of the result- 
ing cells, the three types of the Mendelian ratio might be ac- 
counted for, provided the reduced number of the chromosomes 
is an odd number. If it were an even number, it must often 
happen that equal numbers of a character, of A or B, would be 
contained in the same germ-cell, and hence there would be an 
exact balance, which on the theory should give neither result. 
This, however, is not in harmony with the facts. 
In the light of these theoretical difficulties it seems to me that 
the chromosomal theory must be applied to Mendel’s law with 
caution, and that while at first sight it appears to offer an 
