THE PROCESS OF FERTILIZATION 289 



into a long thin thread which, irregularly intertwined, forms a loose 

 skein, the so-called coil-stage (Fig. 74, B). The thread then begins to 

 thicken, and somewhat later it can be seen to have broken up into 

 a number of pieces of equal length, as if it had been cut into equal 

 pieces with scissors (C). 



These pieces or chromosomes become shorter by slowly contracting, 

 and thus each takes the form of an angular loop, a straight rod, or 

 a roundish, oval, or spherical body (Fig. 74, C, chrs). While this is 

 happening, we can see at the side of the nucleus, and closely apposed 

 to it, a pale longitudinally striped figure with a swelling, similar to 

 a handle, at both ends — the so-called nuclear spindle or central 

 .spindle (ksjy). This is the apparatus for the division of the nucleus, 

 and it was previously represented by a small body susceptible to 

 certain stains — the centrosome, which was surrounded by a halo-like 

 zone, the centrosphere or ' sphere.' This body was long overlooked, 

 but now the majority of investigators assume that, though it is often 

 inconspicuous and very difficult to make visible, it is nevertheless 

 present iu every cell which is capable of division, and that it is 

 therefore a permanent and indispensable constituent of the cell 

 (Fig. 74, A and B, csph). 



When a cell is on the point of dividing, this remarkable cell- 

 organ, which has hitherto seemed no more than an insignificant, pale, 

 httle sphere, now becomes active. First of all, often before the 

 formation of the chromatin coil, it doubles by division {A and B csph), 

 at first only as regards the centrosome, and then as regards the 

 sphere also {B) ; and while division is going on fine protoplasmic 

 filaments issue from the dividing sphere and radiate like rays from 

 a sun into the ceU-substance. As they only retain their connexion 

 with each other at the surfaces of the dividing halves of the sphere 

 which are turned towards each other, we might almost say that fine 

 threads are drawn out between the two halves as they separate, and 

 these become longer the further apart the halves diverge. In this 

 manner the much-talked-of ' spindle figure ' arises, which was first 

 described in the seventies through the researches of A. Schneider, 

 Auerbach, and Biitschli, but the significance and origin of which have 

 claimed the labours of many later investigators down to our own day. 



The processes now to be described do not always take place in 

 exactly the same manner, but the gist of the business is everywhere 

 the same, and it consists in this, that the two ends or ' poles ' of the 

 spindle diverge further and further apart, and between them lies 

 the nucleus whose membrane now disappears {G, km) while the spindle 

 threads traverse its interior. Sometimes the membi-ane is retained, 



I. T 



