144 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



history of the cord, ceased dividing and failed to develop into nerve 

 cells, retaining their embryonic character. Small cells, appai'ently 

 without processes and having nuclei similar to those of the nerve cells, 

 are to be found in the normal brain and cord. The injury to the nervous 

 system is a stimulus which may set some of these cells to dividing. 

 The purpose of cell increase in ganglia many segments back of the 

 region of injury is not apparent. The stimulus of the injury is not 

 restricted to the injured segment, and, in response to it, cell prolifera- 

 tion may occur where it is of no direct advantage in the regeneration 

 of the new nervous parts. 



In conclusion, it is to be observed that the centrosome and the nerve 

 process in the earthworm occupy a definite position in relation to each 

 other and the nucleus. They are always on opposite sides of the nucleus, 

 the centrosome occupying the greater cytoplasmic mass. The point of 

 origin of the process may therefore be considered to be determined 

 at as early a period as the telophase of the last mitosis in the history of 

 the cell. The chromatic mass is the nucleus. The polar region of the 

 spindle occupies the position of the cytoplasmic fundament. The nerve 

 process will be developed from the equatorial side of the nucleus. In 

 the smallest cells that give evidence of any nerve process, the centro- 

 some and process are on opposite sides of the nucleus. 



VI. The Centrosome in Cells of the Epidermis. 



A study of the cells of both the old and the regenerated epidermis 

 brings to light strong evidence for the presence of the centrosome in 

 the resting cell of this derivative of the ectoderm. The cells of the old 

 epidermis are less favorable for the detection of the centrosome than the 

 regenerated cells. The old epidermis consists of much elongated colum- 

 nar cells, whose contents are often coarsely granular. The cells of a 

 recently formed epidermis are more flattened and their contents finely 

 granular. 



In Figure 43 (Plate 6) are shown two cells from the epidermis of the 

 anterior end of a normal worm. In both of these there is a conspicuous 

 deeply stained spherical granule, lying in a clear space directly at the 

 deeper end of the much elongated nucleus. In the cell at the left in 

 the figure this granule (c'so.) is the only prominent object in the cyto- 

 plasm. In the cell at the right, at the outer end of the nucleus, is a 

 region in which are scattered a dozen or more deeply stained granules. 



