CYTOKINESIS. 75 



In certain staoes (e. p-.. fio'. 54) the walls of the alveoles are verv thick and stain so 

 deeph" that I conclude that they are composed of achromatic nuclear material which 

 has diflfused from tlie spheres in addition to the substance of the alveolar layer 

 (hyaloplasm) which is ditfusing toward the spheres from the cell bod}'. These 

 radiations run between the alveoli and not through them, and in certain stages, par- 

 ticularly early prophases and late telophases, show a zig-zag course between alveoli 

 or even branchings and anastomoses around them. figs. 54, 61, el seq. {cf. Wilson 

 '99). 



How to harmonize the well known fact that protoplasm behaves as a thick 

 fluid, with those other well established fiicts as to its differentiations and the local- 

 ization of differentiated structures within it, is a problem to which much attention 

 has been paid. Long ago Briicke ('61) pointed out the fact that a definite organiza- 

 tion could not exist in a fluid and that a fluid plasma would be incapable of per- 

 forming the complex functions of the cell. Since that time the further stud) of the 

 cell has but emphasized the great complexity of its structure and functions, and this 

 has led many authors to regard the cell as a relafivel}^ stable system of parts, the 

 interstices between which are filled with a fluid-like substance. On the other hand 

 the continued study of living protoplasm has more fully demonstrated its fluid 

 character; the freedom with which parts may move about within cells, particularly 

 in certain Protozoa and in metazoan egg cells, is entirely inconsistent with the idea 

 that the cell is traversed by a fixed system of fibres which bind all its parts into a 

 stable system. Biitschli's theor}' of the structure of protoplasm is the only one 

 which undertakes to harmonize these apparently contradictory phenomena, for while 

 emphasizing the fluid character of protoplasm it still assigns to it a definite structure 

 and provides for the local differentiations and specific organization of the cell {cf. 

 Biitschli '92, Rhumbler '98). I accept without reserve the Biitschli theory so far 

 as it concerns the general cytoplasm of the eggs which I have studied, luit I have 

 seen no sufficient evidence that it extends to all the parts of the cell. 



II. Movements of Cell Contents.^ 



In the maturation, fertilization and cleavage of the gasteropod eggs which I 

 have studied I have observed successive stages of a complex and orderly move- 

 ment of the entire cell substance by which the positions of the cytoplasm, yolk, 

 nuclei, centrosomes, spheres and mid-bodies [Zwischeiikorpen) are changed in a 

 definite and orderly w^ay. Unfortunately, I have been unable to actually observe 

 these movements in the living egg, since the eggs studied contain a large amount 

 of yolk and are therefore opaque, and since the movements described are very slow. 

 However, the evidences of these movements are so abundant and unmistakable 

 that one could not be more certain of them if he had seen the actual flowing of the 

 cell substance. 



' A portion of this section was published in the Wood'i Holt Biological Lectures for 1898. 

 (Boston, 1899). 



