402 HARRY H. CHARLTON 



Murray ('98) finds a well-marked resting nucleus in the Pul- 

 monates, Helix and Arion. McGill ('04) found it to happen 

 occasionally in the dragon-flies, and Painter ('14) describes it 

 as occurring in the spiders with the accessory chromosome 

 persisting as a nucleolus. Kingsbury ('01) finds in the salamander 

 Desmognathus fusca, that a nuclear membrane is formed follow- 

 ing the first maturation division, but that the chromosomes 

 never lose their individuality. 



In Lepisma domestica the chromosomes, with the exception 

 of the idiochromosomes, entirely break up and a nuclear mem- 

 brane is formed. While it is undoubtedly of short duration, still 

 the outward individuality of the autochromosomes is lost and 

 the second division is preceded by their reformation. 



The idiochromosomes 



Wilson ('09) divides the sexual differences of the chromosomes 

 into five and possibly seven types. Lepisma domestica falls in 

 line with his type IV in which ''the male has a pair of idiochro- 

 mosomes, half the spermatozoa receiving both and hence two 

 more than the other half. " 



Only one form has been found which has this arrangement, 

 the coreid species Syromastes marginalis L. This form was 

 first described by Gross ('04) and again by Wilson ('09). The 

 accessory chromosome arises by a synapsis of two spermato- 

 gonial chromosomes which divide equationally in the first 

 spermatocyte, but fail to divide in the second. 



Lepisma domestica differs in that the two spermatogonia! 

 chromosomes do not fuse, but remain separate and joined by a 

 stout thread. They pass undivided to one pole in the first 

 spermatocyte division, but separate in the second. Wilson's 

 prediction that the female of Syromastes would have two more 

 chromosomes than the male, he afterward found to be the case. 

 Reasoning in a similar manner, Lepisma domestica females should 

 have thirty-six chromosomes, but unfortunately I have been 

 unable to make any chromosome counts so far in the female. 



