82 



HISTOLOGY AND PHYSIOLOGY. 



each other. In the midtlle they enlarge, and finally coa- 

 lesce to form a nudevs-plate (Fig. 188, n-pl.); the latter 

 divides, and the two halves recede to the poles 

 to form the two nuclei. Then where the nu- 

 cleus-plate originally arose, a cell-plate (Fig. 

 188, c. j)l^ now is formed, and this indicates 

 the partition wall between the two new cells. 



105. The cell-contents.^ — Of 

 the cell products, the chlorophyll 

 (Gr. chloros, green ; phyllon, leaf) is 

 the first in importance. It is the 

 green coloring matter in the cells. 

 In ordinary tissue it is in the form 

 of grains (Fig. 184, c/t) ; in Spiro- 

 gyra it is in the form of spiral hands 

 (Fig. 189) ; and in Zygnenia, another 

 fresh-water Alga, it is in star-like 



Fig. 188. Cell-formalion by division, in case of Spores of Psilotunt triquetntm ; 

 It II, III. IV, V, VI, successive stages; n. pi, nucleus-plate; c.pl, cell-plate. Fig. 

 189. Spirogyra, with chlorophyll in spiral bands. Fig. iOO. Zygnetna, with 

 chlorophyll in stellate masses. 



