298 N. M. Stevens. 



young oocytes, but it is very difficult to find the synapsis stages 

 figured in my '05 paper, PL 16, Figs. 18-25. The chromosomes 

 are extremely small and favorable stages are rarely met with. 



In my first paper on Sagitta ('03) I briefly described the prin- 

 cipal stages in the spermatogenesis of Sagitta bipunctata, show- 

 ing the reduced number of chromosomes to be nine, formed by 

 telosynapsis. Mention was made of the fact that one or two 

 large nucleoli were often seen near one or both poles of the first 

 maturation spindle. In a later paper ('05) on the spermatogen- 

 esis of insects, some additional figures (PL 7, Figs. 226-241), 

 based on further study of the body spoken of as a nucleolus in the 

 earlier paper, were given. This was of especial interest at that 

 time on account of its possible homology with the heterochromo- 

 somes of insects. 



After further study of the element in question in Sagitta ele- 

 gans, new material of Sagitta bipunctata from Pt. Erin and Hel- 

 goland, and considerable work on Sagitta bipunctata, S. minima, 

 and S. inflata, in aceto-carmine preparations, at Naples last year, 

 I am convinced that the elements figured as x in the sperma- 

 togonia, and growth stages of the spermatocytes (PL 7, Figs. 

 226-233) are nucleoli comparable to those described above in 

 oogonia, oocytes and spermatogonia, while the elements so desig- 

 nated (x) in the maturation mitoses (Figs. 235-241) are products 

 of premature division of one of nine bivalent chromosomes. I 

 find some individuals in which the nine chromosomes all behave 

 alike in all or in most cases; while in other individuals, one di- 

 vides prematurely and appears at or near the two poles of nearly 

 every spindle in both first and second spermatocytes. In the 

 first material, which I prepared at Naples and studied at Wiirz- 

 burg in 1902, I found only a very few cases of this phenomenon; 

 so few that I did not publish any figures. The figures in the 1905 

 paper were taken from material obtained from Naples the same 

 year, but used at the tim.e only for the study of the maturation 

 stages of the ova. Sagitta elegans showed the sam.e peculiarity, 

 and the appearance of daughter chromosomes at each pole of the 

 maturation spindle in metaphase was much more frequent. In 

 the Pt. Erin material the difference between individuals in this 



