912 
ganized, but purely chemical aggregates. 
Certain it is that the strands and granules 
and the spongy, knotted structure shown 
in the drawings of the chromosomes by the 
best cytological students of the day agree 
superficially in some respects at least with 
the descriptions of the colloid chemists of 
the structure of two-phased colloidal sys- 
tems. 
On the other hand, our knowledge of the 
chromosomes as independent continuously 
perpetuated organs of the cell has been en- 
larged and confirmed from many sources. 
The extreme form of the doctrine that 
the chromosomes are independent individ- 
ual organisms, cells within the cells, is prob- 
ably an exaggeration, but evidence has 
continually accumulated that the chromo- 
somes are specific and permanent parts of 
the cell which are formed only by the di- 
vision of already existent chromosomes 
and which are nourished and grow after 
division so as to maintain within certain 
limits a size and number characteristic for 
each species. From the cytological stand- 
point also the argument for the significance 
of the serial arrangement of the chromo- 
somes in the splitting spirem has lost none 
of its force. 
The importance of the conclusion that 
each species has its constant number of 
chromosomes has only been emphasized 
by the discovery of cases in which such 
differential qualities within the species as 
distinctions of sex are found to be accom- 
panied by a corresponding difference in 
the number of chromosomes in the two 
sexes. To be sure, such an extreme germi- 
nal difference as the presence of an addi- 
tional chromosome in the female is prob- 
ably a rare case found so far chiefly among 
insects. But the fact that in these cases 
the structural and functional differences 
between the sexes in the adult form are 
correlated with specific differences in 
SCIENCE 
[N.S. Vou. XXXV. No. 911 
chromosome number in the germ cells must 
be regarded as strong evidence that the 
chromosomes represent in some fashion the 
hereditary characters in the germ eells. 
It may well be expected that other slight 
differences in chromosome number which 
have been observed within the species will 
be found to have quite as rational and 
natural an explanation as have the sex 
chromosomes. Other forms of dimorphism 
may perhaps be dependent on correspond- 
ing variations in chromosome number or- 
size. 
In the ease of the sex-chromosomes the 
question as to how the organs and char- 
acteristics of the adult can be represented 
in the germ plasm is raised in a very con- 
erete form. If the presence of an extra 
chromosome determines the female sex, it 
must apparently not only represent the 
specific female sex organs of the adult, but 
also a series of secondary sexual characters 
belonging to the other organs and tissues. 
The difficulties involved in any corpuscu- 
lar theory of heredity become conspicuous 
in such a case. That a series of granules 
representing anlagen for all the various 
characteristics of the female sex should be 
brought together in a single chromosome, 
being thus apparently separated from all 
the other anlagen for the organs which 
show secondary sex characters, and should 
still be able to determine the development 
of the individual in all these respects, is 
searcely conceivable. The discovery of the 
sex chromosomes must be regarded as evi- 
dence against the corpuscular theories of 
heredity. 
The view that characters that are sex- 
limited in heredity must also be deter- 
mined by materials which accompany or 
are included in the sex chromosomes makes 
the situation still more difficult, being quite 
inconsistent with even a serial arrange- 
ment of the elements of the germ plasm 
