279 



in my possession, however, a large number of sections of Arenicola 

 trochophores I examined a number of the early trochophores and found 

 that mitosis was very rare, many sections not containing a single 

 case. On the other hand. Figure 5, Nos. I— X, shows nuclear con- 

 ditions which were very frequent. . All of these cases were drawn from 

 a single section: Nos. 1 — III are from the ectoderm, IV— VI from the 

 mesoderm, and VII — X from the entoderm. Other cases were also 

 present in the same section, which contained no mitoses. Other sec- 



tions and other trochophores exhibited very similar conditions. These 

 observations are of course insufficient to permit general conclusions, 

 but are given merely as another item in the series. 



Sternaspis scutata. In this species the ovary is situated on 

 the wall of a blood vessel. A peduncle containing a loop of the blood 

 vessel which enters the egg is gradually formed as the egg develops. 

 In the regions of the ovary where the primitive cells occur two kinds 

 of cells are found — one with relatively large and pale nuclei, the other 

 with small and deeply staining nuclei. The latter resemble the nuclei 

 which are found along the peduncles, the former the earliest recogni- 

 sable stages of the egg-cells. It is possible that the larger cells arise 

 from the smaller, but I was unable to determine this point with 

 certainty. 



In this form the oogenesis extends over at least five months of 

 the year and the ovaries examined contained egg-cells in all stages 

 of growth as well as many primitive cells. Synapsis stages were fre- 

 quent but no mitoses were observed in these ovaries. On the other 



