434 BIOLOGICAL EFFECTS OF RADIATION 



tissues (Fig. 12). The cells of the old ectodermal epithelium divide 

 mitotically to form the regenerating portion of the nervous system and 

 more epithelium of this sort as the regeneration proceeds. The cells of 

 the endodermal epithelium form more epithelium in a similar manner. 

 In the anterior regeneration of Tuhifex quiescent neoblasts may be 

 present on somites near enough the regenerating region to become 

 involved, as they do in Lumhriculus, but in Tuhifex such neoblasts do not 

 grow and migrate anteriorly (33, p. 438). The new parts are formed by 

 multiplication from the old ectodermal, endodermal, and mesodermal 

 tissues, respectively (Fig. 13). In contrast to this, the anterior regenera- 

 tion of Lumhriculus involves a growth and anterior migration of neoblasts 

 and their division and differentiation in the regenerating end similar to 

 that which occurs posteriorly. The histological changes during the 

 regeneration in both these genera thus proceed from cells within the 

 regions of the respective germ layers in the general manner of embryo- 

 logical development. It is not clear that basal cells of the epithelia are 

 the active units, as Stone (58 and 59) and Turner (63) have stated, 

 since Turner (64) has concluded he was mistaken in this respect and that 

 the "principal cells" of the two epithelia are the ones that multiply by 

 mitosis. For the oligochaetes then, it is clear from the work of Krecker 

 (33, 34) as well as these later investigators and others, such as Hammer- 

 ling (22) and Zhinkin (74), that a definite type of cell plays an important, 

 if not an exclusive, role in the formation of the regenerating mesoderm 

 (cf. Krecker, 34, page 42), and that the epithelia arise in the manner 

 described. 



Reserve cells are also found in many polychaetes such as Filograna 

 implexa and Chaetopterus variopedatus (16 and 17), in which such cells 

 are always present in normal uncut specimens and are the only cells of 

 the body that seem to have embryonic potencies. These can be regarded 

 as homologous with the reserve cells of oligochaetes, since they are similar 

 in origin and since they have a similar part in growth, budding, and 

 regeneration. However, in the sabillid, Euratella chamherlin, Stone (60), 

 found no such reserve cells at any time. In the regeneration of this 

 species,- epidermal cells multiply at various sites and invade the middle 

 region, where they differentiate into the new mesoderm tissues as well 

 as the regenerating nerve cord. As with the oligochaetes, the two epi- 

 thelia are self-restoring. 



The problems indicated by the foregoing regenerative changes in the 

 annulates, particularly in the oligochaetes, include: the origin and 

 potencies of the mesodermal reserve cells; their activation to growth, 

 migration, and differentiation; and the activation to division of the cells 

 composing the two epithelia. The material is favorable alike for gross 

 observation and experimentation in the living state; for study of the 

 histology from whole mounts and sections; and for obtaining quantitative 



