II. 



CYTOKINESIS. 



The term Cytokinesis (Whitman, '87, Ryder, '94, Rhumbler, '96) is here used 

 to designate those movements within the cell body during its cycle of division which 

 correspond to the nuclear activities during the division of the nucleus. Viewed in 

 this light, cell division consists of karyokinesis and cytokinesis, the two being so 

 intimately related that one cannot be treated wholly apart from the other. 



These protoplasmic movements are of particular interest in that they throw 

 light upon the constitution of the cell and the mechanics of cell division as well as 

 upon the more complex problems of differentiation. Thus the movements in the 

 spindle, aster and cell body during division indicate that the protoplasm is of a fluid 

 or semi-fluid constitution, permitting freedom of movement among its parts ; that 

 these movements are in the nature of diffusion streams, as Biitschli has repeatedly 

 maintained; and, further, that these movements are the immediate cause of many 

 important differentiations. 



Before taking up these movements in detail we shall briefly consider : 



I. The Structure of the Cytoplasm. 



The cytoplasm of the egg of Crepidula presents the appearance of being com- 

 posed of alveoles, and this conception of its structure is most in consonance with the 

 many movements within the cell which will be described later. The egg contains a 

 large quantity of yolk in the form of spherules which vary enormously in size. 

 The smaller yolk spherules seem to lie within the alveoles, though the larger ones of 

 course do not. In addition to the yolk spheres which always stain intensely with 

 iron haematoxylin, there are other small spherules which are never seen except in 

 material fixed for a considerable period in Hermann's fluid. These spherules are 

 about the size of the smallest yolk spheres and like them appear to lie within the 

 alveoles, but unlike them do not stain with haematoxylin and are of. a pale gray 

 color. They may represent partially dissolved or digested yolk spheres. 



With the highest powers of the microscope which I have been able to 

 use, viz., Zeiss Apochromatic Obj. 1.5 mm. Occ. 12, the karyokinetic spindles 

 and astral rays show no indication of alveolar structure, nor indeed does the 

 nuclear sap which forms the interfilar substance of the spindle and spheres. 

 In the anaphase and telophase, the spheres as well as the centrosomes are alve- 

 olar or recticular. Within the cytoplasm the alveoles are smaller and their walls 

 thicker the nearer they lie to the centrosomes (figs. 54, 55) as Burger ('92), Eismond 

 ('95), Rhumbler ('96) and Erlanger ('96), have shown to be the case in other forms. 



