264 



XX. STRUCTURE OF THE THALLOPHYTA. 



In Glo&ocapsa polydermatica the cells which have arisen from 

 quite recent division are quite globular (Fig. 101, C). They then 

 begin to grow in length and become elliptic. They then show a 

 slight hour-glass-like constriction (A) in mid-length, after which a 

 delicate partition wall becomes visible at this place, simultaneously 

 with the formation of which the young cells form a delicate wall 



over the rest of their surface. The 

 daughter-cells now round off, and on 

 both arises a relatively thick gelatinous 

 layer, by which they become separated 

 from one another. Thin cell- walls 

 and thicker gelatinous layers thus 

 alternate with one another, and a con- 

 siderable number of generations are 

 therefore combined by the gelatinous 



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ica. in A, at the commence- envelopes into a single cell-tamily or 

 ' colony. By rupture of the outer en- 

 velopes the families fall apart. With 

 careful examination we can in general determine that the number 

 of the thin cell- walls, which their stronger refractiveness has made 

 visible, does not correspond with the number of cell-generations 

 enclosed within. Usually, it is true, the formation of one such 

 cell- wall follows the division of the cell-contents ; nevertheless, 

 two or more such cell- walls can be intercalated in the gelatinous 

 wall between two stages of division. An isolated cell is occasion- 

 ally found, and then is usually surrounded by a considerable 

 number of gelatinous envelopes (Fig. 101, A). In such cases the 

 cell-division is discontinued, not the thickening of the wall. 



FIG. 101. (rliKocapsa polyder- 



