312 ON THE INTERNAL FORM [ch. 



leads to the formation of a net, and the further break-up of the net 

 may lead to the unravelling of a thread or "spireme". 



Here there comes into play a fundamental principle which, in 

 so far as we require to understand it, can be explained in simple 

 words. The eifect (and we might even say the object) of drawing 

 the more permeable body in between the poles is to obtain an 

 "easier path" by which the Hues of force may travel; but it is 

 obvious that a longer route through the more permeable body may 

 at length be found less advantageous than a shorter route through 

 the less permeable medium. That is to say, the more permeable 

 body will only tend to be drawn into the field of force until a point 

 is reached where (so to speak) the way round and the way through 

 are equally advantageous. We should accordingly expect that (on 

 our hjrpothesis) there would be found cases in which the nucleus 

 was wholly, and others in which it was only partially, and in greater 

 or less degree, drawn in to the field between the centrosomes. This 

 is precisely what is found to occur in actual fact. Figs. 90 a and b 

 represent two so-called "types," of a phase which follows that 

 represented in Fig. 89. According to the usual descriptions we are 

 told that, in such a case as Fig. 90b, 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. 90 a, 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 chromosomes, and an ingrowth into the nu- 

 clear area of the "astral rays" — all as in Fig. 91 a, which represents 

 the next succeeding phase of Fig. 90 b. This condition, of Fig. 91 a, 

 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 t." It is clear and obv;ous 

 that the two "types" correspond to mere differences of degree, 



* The spindle is potentially there, even though (as Chambers assures us) it only 

 becomes visible after post-mortem coagulation. It is also said to become visible 

 under crossed nicols: W. J. Schmidt, Biodynamica, xxii, 1936. 



t These v/ere first observed in the egg of a pond-snail (Limnaea) by B. Dumortier, 

 Mim. sur Vemhryoginie des mollusques, Bruxelles, 1837. 



