282 LEONARD P. SAYLES. 



two nucleoli within a single nucleus. This is a phenomenon 

 which is rather infrequent in this species except in regenerating 

 individuals. For brevity, they will be spoken of as "double 

 nucleoli" but this meaning of the words should not be confused 

 with that of earlier workers, particularly Montgomery ('98), 

 whose "double nucleoli" were individual nucleoli made up of 

 two types of material. While no unquestionable cases of double 

 nucleoli have been observed in the first eleven segments, posterior 

 to this region, where the nucleoli are larger, cases of this sort are 

 occasionally found. Counts made on several uninjured indi- 

 viduals indicate that the average frequency is about 3 per 

 segment. 



This larger type of nucleolus is typical of the gut cells for a 

 considerable distance. Then there is a gradual decrease in size. 

 In some individuals it seems to be at considerable distance from 

 the anal segment even 30 segments or more while in others 

 the larger nucleoli apparently persist at least 15 or 20 segments 

 farther. In this posterior portion of the gut they are hardly 

 more than mere dots in the nuclei (Fig. 2). 



Mesoblasts. 



In the ventral portion of the coelom in the growing tail region, 

 cells are present which are presumably derived from the meso- 

 blasts of the developing individual (Wilson, '89 and '92). These 

 cells, which produce the mesodermal structures in the newly 

 forming segments, possess large nuclei (about 4x8/1) and nucleoli 

 (about 2 /x) (Fig. 2). 



Large nucleoli are present, therefore, in the cells of the middle 

 portion of the gut, in those of the hypodermis of the growing tail 

 region and in the mesoblasts. The largest of these are in the cells 

 of the ventral region of the ectoderm, which are forming the new 

 nerve cord, and in the mesoblasts, from which the new meso- 

 dermal structures are derived. The significance of these facts 

 will be discussed under a later heading. 



