RAND : NERVOUS SYSTEM OF LUMBRICID^. 125 



6. Mitosis in the Nervous Fundaments. 



Figures 21 to 30 are taken from the actively dividing cell masses 

 iu the regenerating brain and cord of animals after about five weeks' 

 regeneration. As already mentioned, these cells have very little cyto- 

 plasm and there are no definite cell outlines. Often the nuclei are 

 packed so closely together as to appear imbedded in a common cyto- 

 plasmic mass. In the resting cells of this character I have been unable 

 to discover any structure which could, beyond doubt, be taken for a 

 ceutrosome. If present, it must be an extremely minute body, and all 

 the conditions are most unfavorable for its discovery. It is not until 

 the cell has passed into the earlier stages of mitosis that an unc[uestion- 

 able centrosome is to be observed. 



Cells in the early prophase are numerous and generally present the 

 appearance of the cell at the right in Figure 21. Such cells are con- 

 spicuous objects because of their sharply defined spherical outlines and 

 the clear area surrounding the chromatic elements. The sharp outline 

 is due to the presence of a distinct membrane, and, for reasons to be 

 given later, this membrane must be considered to be a cell membrane 

 and not an expanded nuclear membrane. At some time during the for- 

 mation of the chromosomes the nuclear membrane disappears. 



The appearance of the cells at this stage indicates a condition of tur- 

 gescence. The entire cell contents, cytoplasmic as well as nuclear, with 

 the exception of the chromatin, are in a highly fluid condition, judging by 

 the absence of stained substance (except the chromatin) in the prepara- 

 tions. That there is a swelling of the cell during: the beainninir of 

 mitosis is proved by such conditions as are seen in Figure 21. Here 

 are two dividing cells close together. The one at the right is in the pro- 

 phase, while the one at the left is in the metaphase. (The axis of the 

 spindle in the latter cell is oblique to the plane of the section, and one 

 pole has been cut away. The remainder of the cell was easily identified 

 in the next section.) The two mitotic cells have increased in volume ; 

 and the large nucleus of another cell, caught between them, has been 

 pressed out of shape by the combined pressures from the two swelling 

 cells. 



The smallness of these cells and the large number of chromatic ele- 

 ments make it difficult to determine the exact manner of formation of 

 the chromosomes. One or two nuclei have been found which gave some 

 evidence of a spireme condition of the chromatin, but, aside from these. 



