NUCLEAR CHANGES IN RANA CLAMITANS. 



131 



by amoeboid movement of the cells. Figure 9 shows a section 

 through one side of the cord, in which one layer of cells, not 

 quite at the end, is extending down into the central canal. Up 

 to about six days phenomena such as these may be seen, but 

 sections from six to sixteen days show that the closing is not 

 completed within that period. By sixteen days the new tail is 



Fig. II. Sagittal section through the spinal cord one day after the operation 

 showing the granular leucocytes at the end of the cord, and the pulling apart of 

 nuclei in the lower part of the cord, cc, central canal; Ic, leucocytes. (920 di- 

 ameters.) 



almost as long as it will become (Durbin, 1909), and the spinal 

 cord reaches back close to the epidermis at the posterior end. 

 Still these later preparations show the sides of the neural tube 

 gaping open, and red blood corpuscles extending forward into 

 the central canal of the new cord, as if the pressure of the cerebro- 

 spinal fluid is not sufficient to keep them out. 



4. Cell Division. 

 In an organ such as the spinal cord in which the nuclei lie 

 close together, it is difficult to determine an amitotic division. 

 In order to be sure that amitotic divisions do occur, one must find 

 continuous stages in nuclear and cellular constriction without 

 the formation of chromosomes. Because of the massing of 

 nuclei, this cannot readily be determined in the normal spinal 

 cord, though the slides were examined with this point in mind. 

 The present study gives no evidence that normal nuclei divide 

 amitotically, but stages in direct division can be seen in the 



