130 bulletin: museum of comparative zoology. 



7. Development of the Nerve Cells. 



The significance of these facts concerning the cell membranes will best 

 be uuderstood after a description of Figures 31-36 (Plate 5), which repre- 

 sent further stages in the progress of recently divided cells toward a rest- 

 ing condition. That such objects as are seen in these figures are what I 

 have taken them to be admits, I believe, of no doubt. They occur with 

 about the same frequency as the later stages of mitosis. Moreover, 

 there are no other objects present which could possibly be taken for 

 recently divided cells. They are found in pairs oftener than would 

 occur by chance. That they seem not always to occur in pairs may be 

 explained, in part, at least, by two facts. It is frequently observed 

 that two sister nuclei may progress toward the resting condition with 

 very unequal degrees of rapidity. There is much evidence that this is 

 so in the case of these cells. Figures 32 a and 32 b represent two objects 

 which lay so close together as to leave no doubt as to their being sister 

 cells, but there is a marked difference in the size and compactness of the 

 chromatic masses. It must be that one of two sister cells may so far 

 outstrip the other in regaining the resting condition as to leave the 

 slower cell apparently without a mate. Secondly, when two sister cells 

 lie in different sections, having found one, it is difficult to identify the 

 other, especially if they are not alike in appearance. 



It often happens that two very similar young nuclei are found, not 

 so very far apart, but yet farther apart than they could have been at 

 the end of mitosis. In some regions of the fundaments of the nervous 

 parts the cells are very loosely aggregated, the spaces doubtless being 

 filled with fluid in the living animal. It seems probable that there 

 may be a mechanical shifting of the less securely supported cells, 

 merely as a result of the muscular activities of the animal. In this 

 way young sister cells may become separated from each otlier. 



Figure 31 represents a young cell from the ventral posterior border of 

 the regenerating brain, together with three nuclei of adjacent resting 

 cells. A similar object was found in the next section, but at a distance 

 considerably greater than could have intervened between the two at tlie 

 end of a mitosis. If the darkly stained object in Figure 31 be compared 

 with the chromatic masses and their accompanying polar structures in 

 Figures 2G-30, it will readily be seen that the disappearance in the 

 latter of that part of the spindle which lies between the two chromatic 

 masses, and the assuming of a more nearly spherical form by the chro- 



