rand: nervous system of LUMBRICIDiE. 103 



erated nerve cell is one of the centrosomes of the mitosis of the mother 

 cell, does not necessarily prove that the centrosome of a nerve cell 

 which has arisen by the natural embryonic process of development is 

 also identical with a centrosome of the mitosis of its parent ceil. Yet, 

 if the regenerated nerve cells arise from ectodermal tissues, we should 

 certainly expect the processes of regenerative development to resemble 

 very closely those of embryonic development, especially in so funda- 

 mental a thing as the fate of mitotic organs. Many investigations have 

 shown the similarity of regenerative and embryonic processes. If it is 

 shown that the " centrosome " of a regenerated nerve cell is a true 

 centrosome, — that is, the organ concerned in mitosis, — it must be 

 admitted as highly probable that the similar organ in the naturally 

 developed cell is likewise a true centrosome. 



In an investigation upon any one kind of material, the regenerative 

 process has an advantage over the embryonic, for it affords not only 

 the opportunity for the study of the development of nerve cells, but 

 also the possibility of observing whether the structures under considera- 

 tion in the already differentiated nerve cells have anything to do with 

 the formation of new cells. 



The observations about to be described have been made upon tissues 

 in process of regeneration, the object being twofold, — to determine, 

 first, the behavior of elements in old nervous structures in the presence 

 of the necessity for regeneration ; and, secondly, the source and method 

 of development of the new nervous parts. 



1. Material. 



Two qualities were demanded of material intended for this work. 

 The fully differentiated nerve cells must contain some such structure as 

 has been called a centrosome by various authors, and the animal must 

 have the ability to regenerate excised parts of the nervous system. It 

 was at first proposed to work upon the annelid, Asiothea (Clyme- 

 nella) torquata. In the giant cells of a nearly allied form, Clymene 

 producta, Miss Lewis ('96, '98) found the centrosome and sphere to be 

 present. I found some evidence of a centrosome in the giant cells of 

 Axiothea, but nothing as definite as Miss Lewis demonstrated for Cly- 

 mene. Axiothea was found to regenerate segments very readily at 

 either the anterior or posterior end. I obtained regeneration in a large 

 number of the worms during the summer of 1898, at the laboratory of 

 the United States Fish Commission at Wood's Hole. A number of seg- 



