band: nervous  system of LUMBRICID^. 139 



senting conditions other than these. At v.'hat period of regeneration 

 mitosis of this type appears, I am unable to say. My preparations 

 have nut as yet yielded stages favorable for determining conditions in 

 the very earliest fundaments. In some preparations otherwise favorable, 

 mitosis in the fundaments is rare or absent, indicating tliat the animal 

 was killed during an interval between periods of cell increase. All of 

 the mitoses described are from preparations in which the fundaments 

 are well established, containing, in their deeper parts, cells which have 

 ceased dividing and assumed the appearance characteristic of nerve 

 cells. 



It is not certain that all the products of division, in fundaments at 

 this stage, immediately acquire a large cytoplasmic mass and become 

 differentiated into nerve cells. It is evident that, at some earlier period 

 of the fundament, this could not possibly be so, else the supply of un- 

 ditferentiated cells would be exhausted before the needful number of 

 nerve cells had been produced. It must be that up to a certain period 

 one or both of two sister nuclei acquire only scant cytoplasm after a 

 division, I'etaining their embryonic character for the pm'pose of further 

 division. After a sufficient number of nuclei have been produced, it 

 may be that both of two sister nuclei give rise to nerve cells, while 

 some of the embryonic cells cease dividing without undergoing differ- 

 entiation into nerve cells, remaining as the small embryonic cells seen 

 about the posterior border of the full-grown brain. Or it may be that 

 one of two sister nuclei gives rise to a nerve cell, the other ceasing to 

 divide and remaining as an undifferentiated cell. 



Little can be said as to the origin of the neuroglia of the regenerated 

 brain and cord. As the new nerve cells become differentiated, there 

 also appear among them small nuclei similar in character to nuclei — 

 doubtless of non-nervous nature — found in the normal brain and cord. 

 It is not improbable that cells of the early fundaments give rise to the 

 neuroglia as well as to the nervous nuclei, — a differentiation from the 

 indifferent cicatricial cells in two directions. It is also possible, how- 

 ever, that the neuroglia may he derived from cells having a common 

 origin with those that give rise to the sheath. 



3, Persistence of the Centrosome. 



A definite centrosome is first to be seen at the time of the formation 

 of the spindle in the prophase. It is an extremely minute body at that 

 time, but during the later phases it increases in size, appearing as a 



