HAND : NERVOUS SYSTEM OF LUMBRICID^. 151 



secondarily dividing by constriction. Whatever the proper explanation, 

 the condition is of sufl&cient interest to invite an effort toward obtaininer 

 additional stages in the process. 



X. Summary. 



I. There is commonly present in the nerve cells of Lumbricidae a 

 centred system, consisting of centrosome and radiations. 



1. The single centrosome (or rarely two, or even three, small gran- 

 ules lying close together) is found in the axis of the cell, on the side of 

 the nucleus opposite the nerve process, and therefore on the side of the 

 greatest cytoplasmic mass. It is generally not far from the nucleus 

 and approximately at the centre of the cell as a whole. 



2. Radiations consisting of fibrils bearing minute granules extend 

 from the centrosome toward the periphery of the cell. Calling these 

 ''primary radiations," there may also be distinguished secondary ra- 

 diations, which arise from certain of the larger granules in the course of 

 the primary radiations. In rarer cases tertiary radiations may be found 

 arising from granules in the secondary radiations. The centred system 

 is therefore a complex one, consisting of a chief centre or centrosome, 

 and numerous inferior centres situated throughout the cytoplasm, all 

 with their corresponding sets of radiations, the whole system forming a 

 radiating network whose complexity increases toward the periphery of 

 the cell. 



3. In most cases, no structure which could be called a centrosphere 

 is present. The centrosome, as well as each of the inferior centres, is 

 generally sun'ounded by a small clear space. 



Sometimes the centrosome is surrounded by a narrow region of 

 denser cytoplasm, and the primary radiations, where they intersect the 

 periphery of this region, bear conspicuous granules. In other cases the 

 centrosome lies in a central mass of slightly denser, more finely granular 

 cytoplasm, of perhaps one-half the diameter of the cell, but not bounded 

 by a layer of granules. 



TI. A centred system like that found in nerve cells, of normal worms 

 is found in regenerated nerve cells. Its chief centre, or centrosome, is 

 the centrosome of the last mitosis in the history of the cell. 



1. If the anterior five or ten segments of a worm be removed, the 

 regeneration of a brain and a certain length of ventral cord (not yet 

 segmented) takes place in the course of five weeks. 



