A New Mitotic Structure. By E. Sheppard. 121 



poles in the daughter nuclei, and it will not he out of place if I refer 

 briefly to these two methods. The first, and certainly by far the 

 most frequent, is that in which the splitting commences at the tip 

 of the TJ- or V-shaped chromosome, and gradually proceeds down 

 the entire length of the two arms or phalanges. The other method 

 is that in which the splitting takes place in exactly the opposite 

 direction ; starting at the extreme ends of the arms or phalanges 

 and proceeding up their entire length and finishing at the tip of 

 the U or V. It is understood, of course, that the chromosomes 

 are seen presenting their curved or pointed, and not the bifurcated, 

 ends to the spindle fibres. 



When the two newly separated ends of each pair of chromo- 

 somes make their appearance and are just moving apart from one 

 another, one would quite legitimately ask the question, "What is 

 the initiative force and the action in this process of separation ? " 



To this question we must, to be honest with ourselves, answer, 

 " We do not know," and I can only refer again to the few remarks 

 bearing upon this point I made in the earlier part of this paper, in 

 which I refer to the different theories which have been put forward 

 by various authorities. 



I claim, however, as the result of my work and the application 

 of my methods, that although I am no nearer being able to state 

 what this initiative mechanism or force is in the process of separa- 

 tion and splitting of the chromosomes, I have been able, as 

 careful consideration of my drawings and photographs of those 

 small " chromatin extensions " shows, clearly brought them into 

 view, as the result of successful staining of the spindle fibres ; and 

 it may materially help those who are in a better position than am 

 I, to theorize as to their possible value or purpose in one of the 

 most important processes of mitoses. 



These chromatin extensions are first seen when the ends of 

 the chromosomes have separated but a very short distance from 

 each other at their apices, and the separated ends have a slight 

 but appreciable tendency to turn away from one another towards 

 their respective "poles. As separation proceeds, they become more 

 apparent and pronounced. On careful examination of figs. 1 to 6, 

 PL IV, and abc, text-fig. 15 (drawings from actual cells) small 

 bead-like extensions are seen situated at the extreme tip of the 

 U or V chromosomes. They are very small, and consequently may 

 be easily overlooked unless careful searching be made ; a ^-in. 

 oil-immersion is essential for their correct determination. They 

 take the black stain of the iron-hpeniatoxylin, and with the same 

 intensity as the rest of the chromosome. An important point, and 

 one well worthy of consideration, is that that portion of the spindle 

 fibres (the whole of which stain a beautiful blue) in immediate 

 contact with a bead-like extension of the chromosomes, stains 

 much more deeply than that which is more remote, and we here 



April 21st, 1915 K 



