CYTOKINESIS 199 



the complete details of this work the original memoirs should be 

 consulted. It was found in the root ti])s of both Vicia and 

 Hyacinthus that the nuclear membrane, together with the chromo- 

 somes, first contracted into a " chromosomic pouch " away from 

 the two poles of the original nuclear area after a period of enlarge- 

 ment and left behind it two polar caps of hyaline nuclear sap, 

 which allowed the nuclear membrane to retreat by filtering 

 through it. These caps, which eventually become the spindle 

 cones, were thus topographically nuclear. They were composed of 

 karyolymph, which lay between the shrunken nuclear mem- 

 brane and the cytoplasm. Although there was addition to the 

 karyolymph of cytoplasmic fluid during the time of nuclear 

 enlargement, the cytoplasm contributed no formed element to the 

 developing achromatic figure. Further, according to Robyns, the 

 spindle is optically homogeneous in the natural condition, and the 

 visible fibres and lamellae which can be observed in fixed material 

 are the consequence of improper fixation (Fig. 74). 



CYTOKINESIS 



The differentiation of masses of protoplasm into cells is accom- 

 plished in a number of different ways. In the lower plants the 

 method of separation is as a general rule by means of cleavage 

 furrows or by the accumulation of vacuolar material in special 

 regions and by the formation of cell-plates in the equatorial 

 region of the achromatic figures at the end of mitosis. The term 

 ^^ cytokinesis '^ is given to the differentiation of extra-nuclear 

 protoplasm. 



In the lower Thallophytes, e.g., fungi and algae, the presence of 

 cleavage furrows is a very constant feature of cytokinesis and the 

 process takes place in much the same manner as in animal cells. 

 CEdogonium, however, forms an exception, and a typical cell plate 

 is laid down across the equator of the spindle. In sporocytes, 

 on the other hand, the presence of a cell-plate was always con- 

 sidered to be a characteristic feature. Recently, however, a good 

 deal of attention has been focussed on this particular aspect of the 

 problem and the method of wall formation has been found to vary 

 greatly in different groups. 



