82 



HISTOLOGY AND PHYSIOLOGY. 



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

 lesce to form a mtclem-plate (Fig. 188, n. pi.) ; 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. pi.) now is formed, and this indicates 

 the partition wall l)etween 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, ch) ; in Spiro- 

 gyra it is in the form of spiral bands 

 (Fig. 189) ; and in Zygnema, another 

 fresh-water Alga, it is in star-like .^^ 



Fig. 188. Cell-formation by division, in case of Spores of Psilotttm iriguetrjtm : 

 I, lit m. I^t ^i ^A successive stages ; «. //, nucleus-plate ; r. pi, cell-plate. Fig. 

 189. Spirogyra, with chlorophyll in spiral bands. Fig. 190. Zygnema, with 

 chlorophyll in stellate masses. 



