82 GENERAL PRINCIPLES OF ZOOLOGY, 



named authors made the best observations, the second 

 party was correct in principle. This is explained by the 

 fact that the nucleus in process of division passes into a 

 stage in which it absolutely cannot be recognized without 

 the use of suitable and only recently discovered reagents, 

 because in its place is to be seen only an indistinct lighter 

 spot in the protoplasm. This almost complete disappear- 

 ance of the nucleus was entirely overlooked by the zoolo- 

 gists of one party, but on the other hand it was correctly 

 observed by the other, though falsely interpreted. For the 

 nucleus, as treatment with acetic or chromic acid shows, is 

 still present in the light spot, and has only undergone a 

 transformation into the " nuclear spindle " (Fig. 20). 



Indirect Nuclear Division, Karyokinesis. As the 

 name indicates, the nucleus at the critical stage has assumed 

 the form of a spindle (or often a barrel shape) and the two 

 opposite ends, either pointed or flatly rounded, are the poles 

 of the nucleus (a). The greatest circumference is about 

 midway between the poles, in the equator of the spindle. 

 Here all the chromatin is collected into an "equatorial 

 plate" not, however, forming a continuous mass; on the 

 contrary, the chromatic nuclear substance acquires the 

 form of small granules, or of straight or V-shaped rods, 

 which are called chromosomes. Their number, which may 

 be either small or very considerable, is pretty definite for 

 each form of cell. Diverging from the poles of the nucleus 

 and meeting the chromosomes are delicate achromatic 

 threads, the spindle-fibres, which condition the form of the 

 spindle and play an active role in the nuclear division. 

 That is, each chromosome of the equatorial plate splits 

 into two parts (Fig. 20, b], and under the directing influence 

 of the spindle-fibres the products of the splitting separate 

 and move, one half toward one, the other half toward the 

 opposite pole of the nucleus (c, d, c). 



The collections of chromatin thus arising from the split- 

 ting of the entire equatorial plate are called the daughter- 

 loops. The elements of the daughter-loops again unite 

 with one another and form the two daughter-nuclei. 



