78 CYTOKINESIS. 



as coarse granules — a process ^\'hicll will be described more fully when we come to 

 consider the cleavage. ^ 



What brings the germ nuclei and asters together ? In a former pajaer ('9-i) I 

 suggested that the nuclei were passively drawn together b}' the formation, attachment, 

 and contraction of astral rays, and Kostanecki and Wierzejski ('95) afterward ad- 

 vanced this same view. They maintain that astral rays are strongest while the germ 

 nuclei are being brought together and that as soon as this is accomplished the rays 

 are functionless and disappear. Wilson ('96) regards this view as untenable, and 

 concludes that " the nuclei are drawn together by an actual attraction which is 

 neutralized bj- union, and their movements are not improbably of a chemotactic 

 character." Morgan ('96) also rejects this idea, and I have myself ('99) practically 

 abandoned it. Nevertheless, unless the nuclei are actively locomotive it must still 

 be true that they are brought together by something outside themselves. This 

 something must of necessity be found in the cytoplasm- (including the aster), unless 

 the nuclei are able of themselves to move actively. There is everj- evidence that 

 the nuclei in this, as in most other cases of movement, are passive, and that their 

 movements are brought about by the activit}- of the cytoplasm. 



The migration of the sperm nucleus, like that of the maturation spindles, is 

 accompanied by progressive separation of yolk and cytoplasm, and it is probable 

 that these coincident phenomena have a common cause in general movements of the 

 cytoplasm. 



Furthermore, there are certain elements of constancy in the polar differentiation 

 and in the plane of the first cleavage which cannot be attributed to the nuclei, and, 

 so far as I can see, can be due onl}- to definite characters of the cell bodj-. It is the 

 egg cell rather than the nucleus which shows polar differentiation. The sperm nuc- 

 leus and aster approach the animal pole from various positions ; there is great vari- 

 tion in all the positions of the nuclei and asters relative to each other, and yet there 

 is no variation in the plane of the lirst cleavage which always passes through the 

 point of extrusion of the polar bodies, and in cases where the first cleavage is un- 

 equal the mitotic figure is always eccentric to the same degree. -Now^ the first 

 cleavage, as we shall see, is accompanied by extensive rotary movements of the cell 

 contents, and this fact, joined to the evidences of cytoplasmic movement during 

 maturation and fertilization, leads me to believe that definite movements of cell 

 substance exist in the unsegmented egg. The constancy of cleavage in later stages 

 is associated with constancy of movements in the cytoplasm, and it is probable that 

 the same is true of the constancy which precedes cleavage. 



' A lobe of cytoplasm appears at the vegetal pole just before the germ nuclei meet, fig. 77. It per- 

 sists during the first and second cleavages, being nearly separated from the egg during each division of 

 the cell body, fig. 80. It never divides with the cleavage of the egg, but always remains attached 

 to one of the_ daughter cells, and is gradually resorbed into that cell at the close of the cleavage, 

 fig. 81. This lobe is probably homologous with the "yolk lobe" of Chaetopterus (Mead), of 

 Illyonassa (Crampton), of Fulgur (McMurrich), and of the following gasteropods which I have exam- 

 ined ;_ Urosalpinx, Nassa, Sycotypus and four species of Crepidula. In all the species of Crepidula it 

 contains no yolk and is very small ; in Sycotypus and i^u/f/wr it is larger than in Crepidula and con- 

 tains some yolk, but is still relatively small ; in Illyonassa, N'assa and Urosalpinx it is very large and 

 is filled with yolk. 



