72 PROTOPLASM AND THE AMOEBA. PROTOCOCCUS 



divides into two. The nucleus first divides into two 

 equal parts, and then a furrow appears in the cytoplasm 

 and rapidly deepens till the two halves become entirely 

 separate. This simple method of multiplication is 

 called binary fission. Each of the halves is a complete 

 amoeba in every respect, and each proceeds to lead its 

 own independent life and ultimately grows to the size 

 of the parent amoeba, when it divides again in the 

 same way. [The amoeba has other modes of reproduction 

 which will not be dealt with here.] 

 Protococcus. On the bark of trees, on old palings, 



FIG. 4. Cells of Protococcus viridis. x about 800. Each is bounded 

 by a colourless cell wall (c.w.). The shaded portion is the green 

 chloroplast which frequently occupies most of the space within 

 the wall (ft and c) ; pyr., pyrenoid. In a and d the cells have recently 

 divided and are remaining together. 



etc., especially on the north side, which does not en- 

 counter the direct rays of the sun and thus tends to 

 remain damp, one often finds the surface covered with 

 a thin green crust, which crumbles in dry weather 

 under the point of a knife. If a little of this green 

 crust is scraped off into a drop of water on a slide and 

 examined first with the low and then with the high 

 power, it is seen to consist of numerous very small 

 green cells, which often hang together in groups of 

 two, four, or more. This is a minute unicellular alga 



