154 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



found at the apex of the polar cytoplasmic fundanieut, attaiuiug its 

 greatest size at this period. 



During the early growth of the cytoplasm the centrosome persists, 

 generally being found not far from the centre of the cytoplasmic mass. 

 At an early period evidences of concentric and radiating structure about 

 the centrosome are seen. 



With continued increase of the cytoplasm, the centrosome assumes 

 its characteristic position in the axis of the cell near the nucleus, and 

 becomes associated with a system of radiating fibres whose complexity 

 increases until the conditions found in the mature cell (I. 2) are 

 attained. 



III. 1. Some resting cells of recently regenerated epidermis possess 

 a centrosome and system of radiations similar to those of the nerve 

 cells. 



2. In the mitosis of cells in the regenerating epidermis the centro- 

 some persists in the cytoplasm after the nucleus has regained the resting 

 condition. 



3. In the much elongated columnar cells ^f the epidermis of the 

 normal worm, there is strong evidence for the presence of a centrosome. 

 In these very attenuated cells, however, no radiating system could be 

 detected. 



4. The final stages of mitosis in some cells of the regenerating epi- 

 dermis exhibit peculiar conditions of the cell membranes, pointing 

 toward the conclusion that, after the division of the cytoplasm, a new 

 cell membrane forms within the old cell membrane of each daughter 

 cell, the original cell membrane disappearing. 



IV. 1. The stimulus due to the injury may cause abundant mitosis 

 among the subepidermal cells or basal cells of the epidermis, several 

 segments back of the injured segment. 



2. The centrosome of mitosis in these subepidermal cells persists in 

 the cytoplasm after the cell has i-eturned to the resting condition. 



V. In certain cicatricial cells, evidently of epidermal origin, a few 

 mitoses were found where, in the telophases, both daiaghter chromatic 

 masses were enclosed within a common membrane of doubtful origin 

 (a distinct outer cell membrane being also present). This membrane 

 finally constricts and divides equatorially, becoming, apparently, the 

 nuclear membrane of the two daughter nuclei. 



