blackman: the spermatogenesis of scolopendra. 57 



i. e. parallel to the axis of the spindle. Thus it is evident that the 

 phenomena must he explained in another manner. 



During the separation of the centrosomes the chromosomes are also 

 drawn apart into two groups, and there is seen between these two groups 

 of elements a spindle-shaped structure composed of well-developed 

 archoplasmio fibres. During the elongation of the cell this structure 

 also elongates and at the same time widens considerably (Figs. 68-69). 

 The spindle being placed between the centrosomes, if it exert any 

 force upon these elements resulting in their further separation, must do 

 so by a process of elongation or growth. It is very improbable, I think, 

 that sufficient force could be exerted in this manner to accomplish the 

 elongation of the cell which invariably ensues. I know no single in- 

 stance in animal tissue in which any large amount of work is ac- 

 complished by the lengthening of a fibrillar structure. The reverse is 

 invariably the case when any force is required, for example, the con- 

 traction of the fibrillae in muscle cells. 



Then, it is very probable that the divergence of the centrosomes is 

 accomplished by some force centring in them, i. e. that, while they attract 

 other parts of the cell, they exert a mutually repellent influence upon 

 each other. That organs so very small should possess such great power 

 will not be admitted by many, yet I believe no better explanation is 

 now at hand. The potentiality of these minute structures is shown at 

 various times in the cell's development: in the early prophase, when 

 the nuclear membrane is drawn out into a conical protuberance (Plate 2, 

 Figs. 20, 22) ; later, when it is forced inward (Figs. 21, 29, 30) ; at the 

 time of the disintegration of the nuclear membrane, when the linin fibres 

 are drawn toward, and centre in, this body (Figs. 30, 31 ; Plate 4, Figs. 

 48, 49) ; in the metaphase of the large type, when the spindle revolves 

 through an angle of 90° (Plate 3, Figs. 33-35) ; in the metaphase of 

 the small type, when the chromosomes are drawn to a position between 

 the centrosomes (Plate 5, Figs. 62-64). At all these stages, as well 

 as in the separation of the chromosomes and the cleavage of the cyto- 

 some, the centrosomes show their great influence over the other cell 

 structures. The importance of this cell organ is also shown by its 

 presence at all phases of mitosis and at all stages of cell development. 

 That these structures should behave as they do and still be of no- 

 fundamental use in the cell, is incredible. At all periods of develop- 

 ment the centrosome shows by its behavior that it is of great importance 

 in the cell's economy and that it is indeed one of the most important 

 organs of the cell. That it is probably an organ of equal rank with the 



