MUSEUM OF COMPARATIVE ZOOLOGY. 375 



the same phenomena. Aside from differences in the prominence of the 

 separate features which are to be made out from a comparison of these 

 results, the following variations may be mentioned. The form of the 

 nuclear structure may vary greatly in different objects, from the almost 

 truncate cask-shape to the very pointed spindle, as, for example, in the 

 parent spore-cells of Psilotum (Taf. VI. Figs. 86, 90), or Equisetum (Figs. 

 102, 105, 107) ; but advanced stages usually exhibit in all cases a very 

 plump outline. The nuclear plate may be homogeneous, as though formed 

 by the complete consolidation of its rodlike bodies (Allium, pp. 137, 138, 

 and Taf. VI. Figs. 55, 56); the rods may be few (Taf. VI. Fig. 53), or 

 numerous, large, and closely approximated, as in Psilotum (Taf. VI. Fig. 

 87). The interzonal filaments increase in number and in size, as, for 

 example, in the parent cells of pollen in Allium (p. 1 38) and Tropseolum 

 (p. 140), of spores in Equisetum (p. 149), and of macrospores in Isoetes 

 (p. 158) ; but owing to the smallness of the nucleus, they do not always 

 become convex enough to reach the wall of the parent cell. The gran- 

 ular accumulation in the equator leads to the formation of a continuous 

 structure, the cell plate (pp. 27, 111, etc.), in which is differentiated 

 the cellulose partition of the two new cells. The halves of the cell 

 plate, not receding from each other to any such extent as did the 

 halves of the ^er?zplatte, form the " Hautschicht " of the young cells. 

 Inasmuch as the nuclear structure does not always swell in its equator 

 sufficiently to meet the wall of the parent cell, this cell plate may be 

 supplemented by a similar structure in the surrounding protoplasm 

 (Plattenschicht im Protoplasma, pp. 28, 111, 113, etc.) continuous with 

 it. The adjacent protoplasm may then show a fibrous differentiation 

 (see Taf. 11. Fig. 30) similar to and parallel with that of the nucleus. 

 Thus both nucleus and cell protoplasm may operate conjointly in accom- 

 plishing the formation of the new boundary, or, as in Spirogyra, they 

 may act simultaneously, but separately, for the accomplishment of the 

 same object. Only a single nucleolus makes its appearance in the new 

 nuclei of Ulothrix, while in many nuclei peculiar differentiations appear 

 in the form of granular bands running parallel to the spindle axis (p. 2 7, 

 Taf. II. Figs. 29, 30), or granules are arranged (pp. 96, 119, 138, Taf. V. 

 Figs. 28, 37, and Taf. VI. Figs. 62, 65, 110) in the equator of the new 

 nucleus, in a plane transverse to the axis of the spindle. 



The process of division is somewhat abbreviated in the case of the 

 formation of spore and pollen cells. The nucleus of the parent cell is 

 divided, with the formation of a " Kernplatte," into two, and the " Zell- 

 platte " is indicated ; but before a cell wall can be formed the two new 



