REGENERATION IN LUMBRICULUS. 28 1 



Setigerous Glands. 



Each pair of setae is embedded in a mass of cells derived from 

 the ectoderm and to them are attached secondarily mesodermal 

 cells which form their musculature (Penners, '23). The setee 

 themselves are formed by the ectoderm cells, according to the 

 description of Bergh ('90), more recently corroborated by 

 Penners. In the earlier stages of the formation of the setigerous 

 glands in the growing region of the tail most of the cells possess 

 oval nuclei of about the same size as those of the neighboring 

 ectoderm cells and nucleoli measuring between I and I^M- As 

 muscle fibers are added to the outside of the gland cells it becomes 

 more difficult to study the latter but it seems certain that most 

 of them have large nucleoli for a number of segments from the 

 growing region. In the older part of the worm, however, the 

 nucleoli are smaller, for the most part mere dots (Fig. 3). 

 Occasionally a few cells are to be found possessing fairly large 

 nucleoli (i ju); these cells may have been active in the formation 

 of a new seta to replace one which had been lost. 



Alimentary Canal. 



The cells of the alimentary canal in the first segment are 

 typical hypodermal cells, of cuboidal shape with round nuclei 

 4 to 5 yu in diameter and nucleoli which appear as mere dots 

 (Fig. i). These cells, unlike those of the rest of the digestive 

 tract, are without cilia but covered with a cuticle, as are those 

 of the hypodermis (Pointer, 'n). In the second somite the cells 

 are more elongated and the nuclei are of the oval shape (3.6 

 x 6.9 n) typical of most gut cells of this species. This segment 

 is a transition zone between the first and third as far as the 

 nucleoli are concerned. In the latter the nucleoli are fully \ n, 

 in diameter (Fig. 4). 



These same measurements hold for both the nuclei and the 

 nucleoli in the cells of the next few segments. At about the 

 ninth or tenth segment, however, cells with larger nucleoli 

 appear. These cells increase in abundance until in the vicinity 

 of the thirteenth practically every nucleolus measures at least 

 i /JL (Fig. 5). There is no corresponding enlargement of nuclei. 

 A feature which first appears in this region is the occurrence of 



