308 bulletin: museum of comparative zoology. 



might be difficult to say which of these conditions preceded the other, 

 since there is in the cell itself no definite landmark, the disappearance 

 of the nucleolus occurring at different periods in different cells. When, 

 however, we find that after the disappearance of the nuclear membrane 

 the chromatin is still present in the form of chromomeres, and that 

 these later fuse to form half as many chromosomes, only one interpreta- 

 tion of the method of chromosome formation is open to us. It is, I 

 think, clear that the "segment" stage of the prophase of the sperma- 

 togonia of Gonionemus is not homologous with the segmented spireme of 

 higher Metazoa, although in general aspect it may resemble it, but rather 

 with the spireme itself, which here fails to form a strand of continuous 

 thickness throughout. 



The protoplasmic mass which occupies the position of the nucleus after 

 the disintegration of the latter is, as already noted (Fig. 26), distinctly 

 granular and clearly distinguishable from the surrounding cytoplasm, 

 and since it is in this mass that the spindle rays later develop, we may 

 fairly assume that it is of archoplasmic nature. But whether it consists 

 of persistent archoplasmic granules previously masked by the cytoplasm, 

 and now assembled preparatory to the role they are to play in the en- 

 suing mitosis, or whether it merely represents ordinary cytoplasm altered 

 chemically and morphologically through its contact with the nuclear 

 structures, can not be certainly answered. No doubt very considerable 

 changes in the sui-rounding cytoplasm must occur at the time of the dis- 

 appearance of the membrane, changes affecting both its composition and 

 the degree of local concentration of the colloid substances of which the 

 cell is composed. Although I do not wish to enter here upon any dis- 

 cussion of the much mooted question of the mechanics of cell division, 

 yet such alterations cannot fail to be suggestive, especially in view 

 of the more recent theories of the influence of electrodynamic forces in 

 determining the arrangement of the chromosomes in the equatorial re- 

 gion of the cell. 



]\[etapliase. — After the formation of the dumb-bell-shaped chromo- 

 somes, these rapidly take up a position in the equatorial region of the 

 cell, lying with their long axes parallel to the future plane of division, 

 so that, when the cell is viewed from the side they appear as small in- 

 tensely black dots (Fig. 31). The chromomeres have now become so 

 closely fused that the double outline is usually lost ; and they are rod- 

 like (see Figs. 28, 29 and 30). As soon as they reach the equatorial- 

 plate stage, longitudinal splitting commences, which makes it difficult or 

 impossible to arrive at an accurate count of chromosomes in the early 



