430 BIOLOGICAL EFFECTS OF RADIATION 



NEMERTINA 



Knowledge of regeneration in nemerteans, particularly the genus 

 Lineus, has been greatly extended by the recent studies of Coe (8). In 

 certain species, such as Lineus socialis and L. vegetus, the power of regener- 

 ation "is so great that almost any small piece of the body, provided it 

 contains a portion of one of the lateral nerve cords, is able to develop 

 into a minute worm of normal proportions," and the persistence of this 

 potency can be shown "by repeatedly cutting off portions of partially 

 regenerated pieces until extremely minute individuals less than a hundred 

 thousandth the size of the original are finally obtained." On the other 

 hand, with Lineus pictifrons the ability to restore a new head from a body 

 fragment extends only through the anterior half of the foregut region; 

 while with L. ruber, which is the "broad form" of Nussbaum and Oxner 

 (45), only that part containing the anterior ends of the nerve cords 

 restores a missing head. In addition to his studies of the external and 

 organismal factors, Coe has described the histological factors involved. 

 In an attempt to analyze the organizing potencies he has concluded that 

 the cut nerve cords liberate an agent that activates the dormant cells 

 which are situated in the parenchyma between the organ systems in all 

 parts of the body, and which by virtue of this activation assume the 

 properties of regenerative cells, migrating anteriorly and posteriorly 

 and finally collecting in large numbers beneath the new epidermis which 

 has already covered the cut ends of the fragment. Those that migrate 

 posteriorly are soon incorporated into the organ systems of the original 

 body, which are restored both by the multiplication of the differentiated 

 cells, to form cells of like nature, and by the incorporation of regenerative 

 cells which differentiate into cells of the respective tissues, presumably 

 under the influence of the existing parts. The cells that migrate ante- 

 riorly, on the contrary, form a true blastema consisting of an apparently 

 undifferentiated mass of mesenchyme cells, which, like an early embryo, 

 seems to be a self-determining system composed of cells that are multi- 

 potent and capable of differentiation into any of the new organs. The 

 blastema is thus an essentially new individual. Coe concluded that 

 the different regenerative capacities, or contrasting powers of regenera- 

 tion in closely related species "may be dependent upon differences in the 

 extent of distribution either of the activating agent or of the regenerative 

 cells." 



So far as the writer is aware, the technique of irradiation has not been 

 applied to these problems of regeneration in nemerteans, except by 

 Coldwater (10) whose work has appeared only as an abstract. Whole 

 specimens of Lineus socialis were exposured with 100 kv., 5 ma., 21 cm. 

 and covered by a 75-mm. bakelite filter for periods of 10, 15, 20, and 

 30 min., respectively, which represented approximately 400 r per min. by 



