NEW TYPES OF CHROMOSOME DISTRIBUTION. 121 



as univalents in the first division, but in the second, they come 

 together in the middle of the metaphase plate to form a dyad. 

 The two chromosomes of this dyad group separate, one passing 

 to one pole and the other to the opposite pole. 



Another and the first new type described in the present paper 

 is found in Fitchia, where there are three chromosomes which 

 divide as univalents in the first division, but in the second division 

 arrange themselves in the form of a triad group, two members 

 of which pass to one pole and one to the other. 



In Prionidiis and Sinea, we find a second new type in which 

 there are four corresponding chromosomes. These four divide 

 only in the first division. In the second, they are arranged in 

 the form of a tetrad group, three members of which pass to one 

 pole and one to the opposite one. 



A third new type is present in Acholla rnultispinosa. Here 

 there are six univalent chromosomes in the first division. My 

 evidence is not absolutely conclusive in this case, but as far as 

 it goes, it indicates that these six divide in the first division, but 

 in the second, five of them pass, without dividing, to one pole 

 and one to the other. 



In all these cases the male and female chromosome groups 

 differ correspondingly. In Diplocodus the male and female cells 

 have the same number of chromosomes, but differ in that the 

 female cells have a large idiochromosome and the male cells a 

 small one. In Fitchia, the female has one more chromosome 

 than the male; in Prionidiis and Sinea two; in Gelastocoris three; 

 and in Acholla inultispinosa four. These facts prove that in 

 each case one class of spermatozoa is female-producing and the 

 other class male-producing. 



The discovery of these types adds nothing new in principle 

 to the theory of sex production as put forward by Wilson ('06), 

 but they are perfectly consistent with it. Acholla inultispinosa, 

 in which the male seems to have the larger quantity, offers evi- 

 dence against the interpretation that sex is determined by a 

 quantitative relation of the chromatin. 



In the preliminary note ('08) on Gelastocoris, I used the term 

 " differential chromosomes " to refer to the chromosomes of the 

 pentad group. Throughout the present paper, the same term 



