SUTTON : SPERMATOGONIAL DIVISIONS IN BRACHYSTOLA MAGNA. 139 



The early divisions which result in an increased number of primary 

 spermatogonia I will designate as multiplication divisions, and the 

 last one, which results in secondary spermatogonia, as the transforma- 

 tion division. It is probable that the multiplication divisions cease 

 and a resting stage ensues before any of the cells enter upon the 

 transformation division. 



The chief distinguishing characteristics of the secondary spermato- 

 gonia are the presence of a cyst membrane around the groups in which 

 they are arranged, and the appearance of the distinct chromosomic 

 vesicles in place of the common nuclear membrane. The two-cell 

 cyst, therefore, marks the transformation. 



The transition of the secondary spermatogonia into spermatocytes 

 occurs at the end of the last telophase, preceding the growth period 

 (synaptic stage of Moore), which latter is readily recognized by ^\\e 

 much-increased size of the cells, the large, clear vesicular nucleus 

 with its spireme, and by the characteristic position and shape of one 

 of its chromatic elements (the accessory chromosome). 



This element has been frequently noted by investigators and in a 

 few cases more or less carefull}^ described, but is nearly always re- 

 garded as a nucleolus. Last year, however, McClung. in a paper on 

 "A Peculiar Nuclear Element in the Male Reproductive Cells of In- 

 sects." called attention to its unusual behavior and staining reactions, 

 and demonstrated the necessity for regarding it as an accessory chro- 

 mosome. His investigations, in the main, were on the spermatocytes 

 and spermatids, but my own upon the spermatogonia only serve to 

 strengthen his conclusions. The behavior of this element in the sper- 

 matogonial divisions of Brarhijstola is such that to call it a nucleolus 

 would be ridiculous in the extreme. Therefore, since the name "ac- 

 cessory chromosome," provisionally used by McClung, has proven 

 consistent with all the phases of the element yet seen by me, it will 

 be retained in this paper. Paulmier, also, in a very recent paper on 

 '"The Spermatogenesis of Aiia.sti /yv'.sV/.s-," expresses his ojjinion that 

 this element is not a nucleolus but a modified chromosome, which he 

 designates as the "small chromosome." The membranous investment 

 of the whole follicle will be known as the follicular wall, and that of 

 the separate spermatocysts as cyst membranes (Cysteidiaut of St. 

 George). 



OBSERVATIONS. 



As already mentioned, spermatogonia are of two kinds, primary 

 and secondary. The jjrimary spermatogonia appear in the follicle at 

 its earliest beginning and by their multiplication, by mitotic division, 

 produce a considerable increase in the size of the follicles and con.se- 

 quently of the testis. They are variable in size, are closely packed 



