166 



INTRODUCTION TO CYTOLOGY 



large cell, is divided into uninucleate swarmers by cleavage furrows. 

 These appear to begin their development from the cell and vacuole 

 membranes and from cleft-like vacuoles. Often the furrows thus 

 developing inward from the cell membrane are very narrow. In some 





,:^^^H:^M 





Fig. 92. Fig. 93. 



Fig. 92. — Cytokinesis by furrowing in sporangia of fungi. A, Rhizopus nigricans. B, 

 Phycomyces nitens. {After D. B. Swingle, 1903.) 



Fig. 93. — Cytokinesis by furrowing in Closterium. {After Lutman, 1911.) 



instances, the wall substance seems to be active in developing the furrow ; 

 in such cases a "girdle wall" develops centripetally as a ring-like ingrowth 

 from the sides of the cell (Fig. 94). In Cladophora an accumulation 

 of slimy material, seen against the lateral wall as the process begins, 

 appears to be swollen wall substance. The formation of the girdle wall 





Fig. 94. — A, girdle wall developing in Spirogyra. {After Nathansohn, 1900a.) B-F, five 

 stages in the development of the girdle wall in Cladophora. {After Brand, 1908.) 



is probably independent of the division of the several nuclei, but in 

 Spirogyra such a wall develops immediately after the single nucleus 

 divides and involves the activity of the achromatic figure (McAllister, 

 1931). The "division by constriction" seen in unicellular alga?, the 



