CELL-DIVISION. 115 



that a delicate film or plate is produced, separating the two 

 halves of the spindle from one another ; concomitantly with this 

 change, the whole spindle contracts somewhat towards the 

 median plane, and this contraction has the result of bulging out 

 the circumference of the spindle, so that ultimately it touches the 

 side walls of the cell. In this manner the cell-plate comes to 

 extend right across the cell and constitutes the rudimentary 

 partition-wall separating the two resulting cells from one another 

 (see Fig. 90). The cell-plate is in its later stages composed of 

 chemically pure Cellulose, and it has been shown that during its 

 formation the protoplasm (or kinoplasm) becomes directly con- 

 verted into cellulose by the splitting off of its proteid and 

 amine portions. 



The two halves of the spindle lying on either side of the cell- 

 plate persist, and the achromatic fibrils become ultimately con- 

 verted into bridles of cytoplasm, which communicate with one 

 another through minute "pits" in the partition-wall. 



The origin of the achromatic spindle is somewhat hypothetical; 

 thus it has been supposed to arise from the kinoplasm, just out- 

 side the nuclear membrane, but at times it seems that it takes its 

 origin from the nuclear plasm. Some observers state that the 

 spindle-fibres arise early and lie as a sort of feltwork just outside 

 the nucleus, which, as mitosis proceeds, pushes its way into the 

 interior of the nucleus towards the chromosomes. In Stypocaulon 

 and Erysiphe, according to Harper, the spindle is an intra-nuclear 

 formation, so that here it would appear to arise from the 

 nuclear-plasm. 



In those cases where, as in lower plants, centrosomes are 

 formed in the cell, the achromatic spindle is an early formation, 

 and arises between the centrosomes, close to the nucleus, during 

 the early prophase. Moreover, fibrils, known as the " astral 

 rays," stretch out in all directions from the poles or centrosomes, 

 and not only into the interpolar region. 



With regard to variation in form of the chromosomes, it 

 may be mentioned that at times the chromosomes may take on 

 the form of rings instead of loops, the free ends of the loops 

 remaining united for some time. This is known as heterotypic 

 mitosis, the process above described being normal or homotypic 

 mitosis. Moreover, it is an interesting fact, that during the 

 reproductive divisions in the microspore and embryo-sac of 



