D. B. WARD, M. D. 



73 



becomes two amoebas. Thus there is a sort of immor- 

 tality about it, for each amoeba must contain some por- 

 tion of all its ancestors back to the original amoeba. 

 There is a familiar unicellular plant called the protococ- 

 cus viridis which grows in the late fall and winter on the 

 north side of the trunks of trees, coating them with a 

 brilliant green whenever the weather is wet. This little 

 plant has about the size of the human blood cell on the 

 average. Each cell splits in two, then four, and so on, 

 thus accounting for the extremely rapid growth which 

 covers the bark in a few hours with a thick and brilliant 

 coating of green. Now the diatom multiplies in the 

 same way by fission or splitting ; but as it is obvious that 

 if it split in two in its ordinary state it would then be in 

 the condition of an oyster on the half shell, a modifica- 

 tion of the process is necessary. This modification is 

 showD in figures 1, 2, 3, 4. 



o 











O 



o 







The hoops slide over each other and the diatom becomes 

 thicker, the nucleus becoming elongated and the nucleo- 

 lus split in two. Then the nucleus itself splits and a 

 new valve is formed from each hoop, as shown by the 

 dotted lines. In Fig. 4 the process is complete and the 

 two diatoms have separated, each one having a new and 

 an old valve. After a certain number of repetitions of 

 this process the vitality of the plants seems to become 

 impaired. They have as it were " run out" and become 

 small and feeble, when a new process restores the race to 

 its full size and activity. This is the process of conju- 

 gation with formation of spores. Two diatoms coming 



11 



