TimhcriaL'c — Sivarm-S pores of Ilydrodidyon. 505 



the cytoplasm. (Fig. 22.) Seen in a surface' section these 

 furrows appear as single lines and thus might easily he taken 

 for cell plates formed in the protoplasm without the help of 

 visible spindle fibres ; but if a vertical section is studied, the 

 appearance of furrows becomes manifest. (Figs. 31-32.) 



In these latter sections it is also clearlv shown that the 

 cleavage furrow'S thus formed cut through at right angles to 

 the surface. In none of mv material have I found the fur- 

 rows forming appreciable oblique angles with the surface, al- 

 though such a phenomenon might of course occur in large cells 

 in which the protoplasmic layer is much thicker, as for exam- 

 ple, in such a cell as that shown in Fig. 2. I have not suc- 

 ceeded in getting the stages of cleavage in these larger cells. 

 During the succeeding stages the furrow^s become branched and 

 increase in length so as to soon intersect with one another and 

 thus block out very irregular multinucleate areas on the surface 

 of the protoplasm. Concurrently with the development of the 

 cleavage furrows on the outer surface similar ones are formed 

 by the vacuolar membrane on the inner side of the protoplasm 

 (Fig. 33.) The two furrow^s from opposite sides finally meet 

 in the interior of the protoplasm and fuse, thereby completely 

 cutting through the entire layer. The process of growi:li and 

 branching of the cleavage furrows continues until the entire 

 protoplast is cut into unincleate pieces, which later round up 

 into the sv^-armspores. 



While the tw^o sets of cleavage furrows generally seem to ac- 

 complish the complete division by coming together from oppo- 

 site piaces on the two surfaces, it frequently happens that one' 

 furrow may reach nearly through the entire layer of proto- 

 plasm before there is any indication of a corresponding furrow 

 being formed on the opposite side. Still I have not found 

 any cases wdiere one furrow^ cut clear through without meeting 

 a constriction from the opposite side. The deeper constric- 

 tion may, how^ever, be from, either the inner or outer surface. 

 In the early stages of cleavage the constrictions are irreg- 

 ular and wavy in a surface section, while in the later stages 

 they seem to become straighter and their intersections form 

 sharper angles. (Fig. 27.) 



