No. 3.] THE TAIL IN LUMBRICULUS. 325 



presence and the position of the neoblasts in preparations made 

 by opening the worm so that the inner surface of the body-wall 

 could be examined (Fig. 2), and have also found them regularly 

 recurring in series of transverse sections, in alternation with 

 the successive setae. 



About the end of the first day the ventral space within the 

 proliferated ectoderm is occupied by several large cells lying- 

 free and in process of division (Figs. 3 and 4). At first the 

 products of division are conspicuously large. In consequence 

 of the great size of the neoblasts their division, especially at 

 first, takes place very slowly. To this, I think, is due the fact 

 that one or more are so generally found in some phase of cell 

 division. The reverse is, perhaps, true for the cells of ectoderm 

 and entoderm, although here the much smaller size doubtless 

 adds greatly to the difficulty of distinguishing the stages of 

 division. 



At a somewhat later period the new cells thus formed are 

 smaller, and only a few neoblasts are to be found scattered 

 among the otherwise uniform tissue. The cells that arise in 

 this way eventually occupy the greater part of the ventral and 

 lateral space between the ectoderm and entoderm, reaching 

 dorsally on each side the level of the dorsal setae (Fig. 8, x). 



In very early stages, as soon as the ectoderm and entoderm 

 have extended themselves sufficiently to form a new cavity, 

 there are present — dorsally, laterally, and ventrally — small 

 cells that seem to be wholly unconnected with the neoblasts 

 and their products (Figs. 3, 5, 6). They are much smaller, and 

 appear in the earlier stages when the neoblasts are few in num- 

 ber and before their division products have become in any way 

 differentiated. Of the source of these cells I am not entirely 

 sure. One very clear set of early sections, however, shows with 

 great distinctness cell division taking place in the region of the 

 dorsal peritoneum just at the posterior limit of the old tissue 

 (Fig. 5, /). Upon careful examination I do not find any con- 

 nection between these small mesodermic cells and the ectoderm 

 and entoderm. I infer, therefore, that the regeneration of the 

 dorsal mesoderm is similar to that of the ventral, but I believe 

 that the tissue in the two regions arises separately. 



