IV] STRUCTURE OF THE CELL 179 



E. B. Wilson's*), we are told that, in such a case as Fig. 44, the 

 "primary spindle" disappears and the centrosomes diverge to 

 opposite poles of the nucleus; such a condition being found in 

 many plant-cells, and in the cleavage-stages of many eggs. In 

 Fig. 45, on the other hand, the primary spindle persists, and 

 subsequently comes to form the main or "central" spindle; 

 while at the same time we see the fading away of the nuclear 

 membrane, the breaking up of the spireme into separate chromo- 

 somes, and an ingrowth into the nuclear area of the "astral rays," 

 — all as in Fig. 46, which represents the next succeeding phase of 

 Fig. 45. This condition, of Fig. 46, occurs in a variety of cases; 

 it is well seen in the epidermal cells of the salamander, and is 

 also on the whole characteristic of the mode of formation of the 

 "polar bodies." It is clear and obvious that the two "types" 

 correspond to mere differences of degree, and are such as would 

 naturally be brought about by differences in the relative per- 

 meabilities of the nuclear mass and of the surrounding cytoplasm, 

 or even by differences in the magnitude of the former body. 



But now an important change takes place, or rather an 

 important difference appears ; for, whereas the nucleus as a whole 

 tended to be drawn in to the stronger parts of the field, when it 

 comes to break up we find, on the contrary, that its contained 

 spireme-thread or separate chromosomes tend to be repelled to 

 the weaker parts. Whatever this difference may be due to, — 

 whether, for instance, to actual differences of permeabihty, or 

 possibly to differences in "surface-charge," — the fact is that the 

 chromatin substance now behaves after the fashion of a "dia- 

 magnetic" body, and is repelled from the stronger to the weaker 

 parts of the field. In other words, its particles, lying in the 

 inter-polar field, tend to travel towards the equatorial plane 

 thereof (Figs. 47, 48), and further tend to move outwards towards 

 the periphery of that plane, towards what the histologist 

 calls the "mantle-fibres," or outermost of the lines of force of 

 which the spindle is made up (par. 5, Fig. 47). And if this com- 

 paratively non-permeable chromatin substance come to consist of 

 separate portions, more or less elongated in form, these portions, 

 or separate "chromosomes," will adjust themselves longitudinally, 



* The Cell, etc. p. 66. 



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