224 



THE CELL 



II. Partial or Meroblastic Segmentation. 



III. So-called Free Cell-Formation. 



IV. Division with Reduction. 



The most instructive examples of the various methods of cell 

 division are afforded, for the most part, by animal egg-cells ; for 

 here the divisions follow so quickly one upon another, that the 

 normal conditions may be clearly observed. 



la. Equal Segmentation. 



In equal division the egg, if, as is generally the case, it is 

 spherical, is first split up into two hemispheres. According to- 

 the law explained above, in the division which follows, the nuclear 

 spindle must place itself parallel to the base of the hemisphere, so 

 that the latter is divided into two quadrants. Further, the spindle 

 axis must coincide with the longitudinal axis of each quadrant, so 

 that in each case a division into two octants is produced. In con- 

 sequence, during the second and third stages of the cleavage 

 process, the relative positions occupied by the second and third 

 division planes towards one another, and towards the first division 

 plane, are strictly according to law ; that is to say, the second 

 cleavage plane cuts the first at right angles, and halves it, whilst 

 the third is perpendicular to the two first, and passes through the 

 centre of the axis in which they intersect. If now the ends of 

 this axis are considered as the poles of the egg, the two first 

 division planes may be regarded as meridional, and the third as- 

 equatorial. 



In many cases, after the second cleavage, the four portions may 

 be seen to separate somewhat from one another, the result of 



which is that the furrows produced by 



the second division no longer intersect 



in one point, but meet the first formed 



meridional furrow at a little distance 



from the pole (Fig. 113). Thus a 



transverse line, the cleavage line, which 



varies in length, is produced. I have 



found this especially well marked (VI. 



30b) in the eggs of Sagitta (Fig. 113). 



A short time after the termination 



Fig. 113. a four segmented of the second division of the egg of 



egg of Sagitta seen from the Saqittci, the four cells so arrange them- 



animal pole, (x 160; Hertwig, 



Pi. v., Fig. 5.) selves (Fig. 113) that only two ot them 



