196 A. C. WALTON 



one-half of the daughter cells, while all the other chromosomes 

 underwent division. 



The longitudinal plane of the second spermatocytic division 

 passes through the idiosome, attached or free, and clearly points 

 to the fact that this sex chromosome must be of the X-type. 



This behavior is in complete accord with the known series of 

 events occurring in A. megalocephala where the X- type of idio- 

 some is attached to the end of one of the autosomes, and there- 

 fore it is only logical to apply the same interpretation to A. 

 felis and call the sex chromosome one of the X-type, and similarly 

 attached to the end of an autosome. 



Wilson ('10) has shown that in many insects the idiochromo- 

 some unit appears as a pseudoplasmosome during the growth 

 period of the spermatogonium, distinctly separate from the 

 chromatin masses from which the autosomes appear. Wilson 

 has also shown that the 'Y' component of a sex-chromosome 

 complex probably has little if any part in sex determination, 

 oftentimes not appearing at all in the germ cells of certain indi- 

 viduals of species which generally show its presence. Wilson also 

 states that in certain insects the various individuals may have 

 the same number of chromosomes, but that these chromosomes 

 may not always be of homologous types, though by any but the 

 most careful examination of large amounts of material the differ- 

 ence may not be detected. Such a possibility in the material 

 of A. felis must not be overlooked, and perhaps may account for 

 the difference of interpretation between Edwards and Boveri and 

 between Edwards and the writer. The same phenomenon of 

 variation probably also accounts for the divergences from the 

 normal in the behavior of certain chromosomes in some of the 

 germ cells. 



