kand: nervous system of lumbricid^. 123 



in the cells of the fundaments, and to find, if possible, a series of cells 

 which shall represent successive stages in the development of nerve cells. 



Preparations from the later stages of regeneration were found most 

 favorable for this pui'pose. In worms of from thirty to forty days' 

 regeneration the new brain and ganglia were represented by large 

 masses of cells, among which dividing cells were very abundant. In 

 the same animals the differentiation of nerve cells was in progress, and 

 frequently a single section contained not only dividing cells with scant 

 cytoplasm, but also large, apparently fully developed nerve cells, and 

 perhaps many intervening stages. 



In the earlier stages of regeneration, cells in a solid mass, showing no 

 signs of segmentation, are found lying laterally and venti'ally about the 

 fibre bundle which has previously marked out the position of the nerve 

 cord. In the brain fundament a similar mass of cells lies about the 

 neuropile, chiefly on its dorsal and posterior border, as seen in Figure 18 

 (Plate 3). This position of the cells is characteristic. These cells have 

 a very small amount of cytoplasm, so that the fundaments look like 

 masses of solidly packed nuclei. There are no distinct cell outlines, 

 but where the nuclei ai-e less solidly aggregated an irregular mass of 

 cytoplasm may be found collected about each one. At sufficiently early 

 stages the nuclei are all alike, ellipsoidal or spherical in form, and gen- 

 erally have a single large spherical nucleolus. Nuclei without a nucleo- 

 lus are often found. They may belong to cells which have recently 

 divided. The absence of the nucleolus is often associated with a con- 

 dition of the chromatin which indicates a recent or approaching 

 division of the nucleus. 



At later stages of regeneration many of the nuclei lying deeper in the 

 mass become larger and more nearly spherical and accumulate a con- 

 siderable body of cytoplasm about them. Cells which assume this charac- 

 ter have ceased dividing and are in process of development into nerve cells. 

 The nuclei on the outer borders of the fundaments retain their em- 

 bryonic character and continue actively dividing. 



The more advanced the stage of regeneration, the more nearly have 

 the deeper cells attained the character of typical nerve cells. 



Figure 20 (Plate 4) represents a parasagittal section through the brain 

 of a worm after thirty-four days' regeneration. The regenerated brain is 

 smaller than the normal brain. (Compare Figure 20 with Figure 19, a 

 similar section from the corresponding region of the brain of a normal 

 animal.) The cells occupy the posterior and dorsal border of the nexi- 

 ropile. This relation of cells and fibre mass is found at the earliest 



