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. 



MendeVs 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 oogonia, the 

 number of chromosomes is the same as the number in the 

 body-cells, i.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, i.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. 



