124 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



stages iu the fundament (Figure 16) and persists through all the later 

 development. It is characteristic also of the normal brain. (Compare 

 Figure 19.) In the deeper part of the cell mass (Figure 20) may be 

 seen a number of pear-shaped cells with their processes directed toward 

 the centre of the neuropile. Tliese cells are, to all appearance, fully 

 differentiated nerve cells, like those occupying similar positions in the 

 normal brain. Between these cells and the periphery are smaller cells, 

 some of them pear-shaped and with processes, others in which no pro- 

 cesses could be found. About the extreme posterior border is a mass of 

 nuclei with indefinitely assignable cytoplasm and exhibiting abundant 

 mitoses, — exactly the condition which the entire cell mass of the fun- 

 dament presents at earlier stages. 



It is evident, then, that the deeper cells are the first to become differ- 

 entiated into nerve cells, while the cells on the periphery of the mass 

 long retain their embryonic character and continue dividing to give rise 

 to new elements. 



In the normal brain (Figure 19) the large typical nerve cells occupy a 

 deep position. About the posterior border of the brain are smaller cells, 

 some pear-shaped and with processes, others apparently lacking pro- 

 cesses. There are also nuclei like those of the smaller pear-shaped cells, 

 but having very scant cytoplasm, or there may be several of them lying 

 near together in what is, to all appearance, a common cytoplasmic mass. 

 Such nuclei, or groups of nuclei, differ in no visible way from the ac- 

 tively dividing nuclei which constitute the early fundaments. They 

 resemble likewise the peripheral layer of nuclei present at so late a 

 stasre of res-eueration as is seen in Figure 20. If it could be assumed 

 that the embryonic development of the normal brain is similar to the 

 regenerative development, there would be little reason for doubting that 

 the small cells and " indifferent " nuclei of the posterior border of the 

 normal brain represent the mass of embryonic cells which have given 

 rise to the differentiated nerve cells. There is little or no reason for 

 regarding them as neuroglia cells. The nuclei which, beyond question, 

 belong to the neuroglia are of an entirely different character. 



Conditions in the regenerating cord are similar to those in the brain. 

 The deeper cells are first differentiated into nerve cells, while the cells 

 along the ventral border and at the anterior tip of the ganglionic mass 

 continue dividing long after the deeper cells have attained the size and 

 form of typical nerve cells. 



