xi] IDIO-CHROMOSOMES 159 



Forficula, Locusta, Gryllus, Stenobotbrus, etc.), the Orders in 

 which it was first observed, it has been found in many 

 Insects of the Orders Diptera, Coleoptera and Neuroptera, 

 in Myriapods, in some Spiders, in a number of Nematode 

 worms, and in several Mammals, including Man. A some- 

 what similar condition has also been described in one or 

 two Molluscs and in a Trematode (Schistosoma = Bilharzia). 

 It is thus a phenomenon of widespread occurrence, but, as 

 will be seen later, there is evidence that it is not by any 

 means universally true that sex is determined by the 

 presence or absence of a particular sex-chromosome in the 

 spermatozoon. In the groups in which this condition is 

 found, even when the "X" and "2^" chromosomes of the 

 male are alike, they can usually be distinguished from the 

 other chromosomes (sometimes called autosomes} by their 

 different behaviour in the spireme stages and also not infre- 

 quently by the fact that in the maturation mitoses they 

 divide and pass to the poles either rather before or rather 

 after the ordinary " autosomes." Most frequently the idio- 

 chromosomes lag behind the autosomes in the spermatocyte 

 anaphases, and the* presence of such a lagging pair has 

 sometimes been the first observed indication of the exist- 

 ence of a pair of idio-chromosomes. Another difference 

 between the idio-chromosomes and the autosomes that is 

 characteristic of the Hemiptera, but not usually found in 

 other groups, is that the two idio-chromosomes divide 

 equationally in the first spermatocyte mitosis and then pair 

 and separate to opposite poles in the second. The auto- 

 somes in this Order, as elsewhere, pair and separate in the 

 first division and divide equationally in the second. 



In some species of Insects the condition is rather more 

 complicated than in the examples described above. Several 

 cases have been described in which the male has a multiple 



