THE "ACCESSORY CHROMOSOME" OF ANASA 



TRISTIS. 



KATHARINE FOOT AND E. C. STROBELL. 



In the Quart. Jonrn. Mic. Set., Vol. 48, 1905, Professor]. E. 

 S. Moore and L. E. Robinson writing on the spermatogonesis of 

 Periplanata Americana claim that the nucleolus of the first 

 spermatocyte is undoubtedly the homologue of the structure de- 

 scribed by Paulmier ('99), Montgomery ('01), and McClung ('02), 

 in different forms as one or two of the spermatogonial chromo- 

 somes. It's morphological resemblance to a chromosome shown 

 by its frequent elongate form, Moore and Robinson attribute to 

 mechanical influences and claim that normally it is spherical like 

 the nucleolus. In three recent papers Professor E. B. Wilson 

 ('05, '06) has given special attention to this structure in a num- 

 ber of forms and the above interpretation of Moore and Robinson 

 he ascribes to superficial work. 



A study of the spermatogenesis of Anasa tristis 1 has convinced 

 us that for this form the interpretation of Moore and Robinson 

 is correct, that the nucleolar-like structure of the rest stage is 

 the homologue of the nucleolus of the egg, that it is not a chro- 

 mosome, as claimed by the three cytologists who have investi- 

 gated this form. 



In 1899 Paulmier identified this body of the rest stage with 

 the two small spermatogonial chromosomes which Wilson has 

 aptly named the " microchromosomes." 



Montgomery in 1901 supported Paulmier in this interpreta- 

 tion, but in 1906 he changed his position and now supports 

 Wilson in identifying this structure the so-called "chromo- 

 some nucleolus " as one of the larger spermatogonial chro- 

 mosomes an unpaired spermatogonial chromosome, called by 

 Wilson the odd or " heterotropic " chromosome. Both these in- 

 vestigators claim that it divides only in the first division, in the 

 second division, passing undivided to one pole of the spindle. 



1 We are indebted to the courtesy of Dr. P. R. Uhler for identifying our material. 



