72 Experimental Zoology 
ters are involved. Mendel found that the results with three 
characters agree also with the expectations. As the number of 
characters increases further, the results will be very complex 
and difficult to detect except by an exhaustive series of experi- 
ments, although each single character can easily be traced and 
found to follow the Mendelian law. Under these circumstances 
we might anticipate that types differing in many characters 
would give results too complicated for analysis, especially if 
some of the characters follow Mendel’s law and others follow 
other laws of inheritance. The generally accepted statement 
that species hybrids are intermediate in character between 
the parental types does not appear to hold in all cases critically 
examined for all the characters. It is evident that in the future 
the heredity of each character must be studied by itself. 
Mendel’s Law and the Germ-cells 
On the assumption that the characters of the animal or plant 
are represented by primordia or elements or unit-characters in 
the chromosomes, the following attempt to account for the 
purity of the germ-cells, assumed on Mendel’s hypothesis, has 
been suggested by Sutton. 
In the early germ-cells, the spermatogonia and odgonia, the 
number of chromosomes is the same as the number in the 
body-cells, z.e. the somatic number; but just before the two 
maturation divisions there is a synapsis stage, in which the 
chromosomes come into closer connection with each other, 
and, as Montgomery has shown, it is probable that at this 
time the chromosomes pair with each other in such a way 
that each paternal chromosome unites with its homologous ' 
maternal chromosome; and for the working out of Sutton’s 
scheme it is essential that each paternal unites with its 
homologous maternal, 7.e. that the paternal do not unite with 
any other maternal or with each other. 
* Homologous chromosomes are those that have the same form, or, according 
to some writers, similar characters. 
