72 Edmund B. Wilson 



type that is not yet known to be exactly paralleled in other 

 forms; though, as will appear, the genus Galgulus presents a 

 somewhat analogous case. It does not seem to have occurred to 

 Dr. Gross (as it did not to me until I had carefully studied both 

 forms) that Syromastes and Pyrrochoris might be of different 

 type, but such is evidently the case. I shall endeavor to show that 

 Pyrrochoris is of quite orthodox type, having an odd somatic 

 number in the male and a typical unpaired idiochromosome. 

 Since I am compelled to differ with Dr. Gross in regard to this 

 species, I am glad to admit that the doubts I formerly expressed 

 as to his account of the spermatogonial number in Syromastes, 

 were unfounded. In regard to the female number, on the other 

 hand, I believe he was misled by a wrong theoretic expectation 

 (as he evidently was in case of the male Pyrrochoris) , though it 

 is possible that his determination of the apparent number was 

 also correct, as indicated beyond. 



SYROMASTES MARGINATUS L. 



Gross's account of this form was as follows: The somatic 

 groups in both sexes are stated to show twenty-two chromosomes. 

 The " accessory" chromosome arises by the synapsis of two 

 spermatogonial chromosomes, and is therefore a bivalent. It 

 divides equationally in the first spermatocyte division but fails to 

 divide in the second, passing bodily to one pole in advance of the 

 other chromosomes without even entering the equatorial plate. 

 All of the spermatid-nuclei thus receive ten chromosomes and 

 half of them in addition the "accessory." These are the essen- 

 tial conclusions; but they are complicated by the following singular 

 view of the relations between the "accessory" and the micro- 

 chromosomes or "m-chromosomes." The chromosome nucleolus 

 of the growth period is supposed not to give rise (as it does in 

 Pyrrochoris and other forms) to the heterotropic or "accessory" 

 chromosome of the spermatocyte divisions, but to the m-chromo- 

 some bivalent the same view as the earlier one of Paulmier 

 ('99) which has since been shown to be erroneous (Wilson '05 c). 

 But, on the other hand it is believed to arise, not from the 



