6 ORGANIZATION AND LIFE-HISTORY OF PROTOZOA 



certain cells acquire the power of continuous and rapid multiplication, 

 so much so that they become to all intents and purposes parasites, which 

 bring about the death of their host. These cells can be inoculated from 

 animal to animal indefinitely, and in them they will continue to multiply, 

 just as trypanosomes do in successive passages in experimental animals. 



An ovum, according to the non-cellular view, is a non-cellular indi- 

 vidual, which at once becomes cellular when segmentation occurs. The 

 cells, each of which gives rise to only part of the individual which will 

 normally develop from the ovum, are nevertheless potential individuals 

 themselves, as is demonstrated by the fact that if the cells are separated 

 from one another artificially, as in the well-known experiments with sea- 

 urchin eggs, each is capable of giving rise to a complete embryo. 



It seems evident that the cells of higher animals are capable of 

 independent life provided the proper environment exists. Under 

 natural conditions all the cells of the body contribute to the production 

 of this environment, which is so delicately balanced that separated and 

 isolated cells invariably die unless the proper environment is present or 

 is artificially provided, as in the culture experiments just mentioned. 

 If the environment necessary for the continued life of cells in the body 

 can be kept constant, the cells will survive and reproduce indefinitely, 

 but if some of the cells fail to fulfil their part in the production of this 

 environment, the other cells will suffer and death will result. It may be 

 said that any single cell of a Metazoon is living in a condition of symbiosis 

 with all the other cells. Without entering further into the discussion, 

 for purposes of this work it is sufficient to follow the more orthodox view 

 and to regard the Protista as unicellular organisms, or single cells which 

 still lead a completely independent existence, and the multicellular 

 organisms as groups of cells which work together for a common end. The 

 latter have become so completely interdependent that their power of 

 separate existence has been largely lost. Yet in many features, such as 

 their structure, mode of life and method of reproduction, nuclear division 

 and syngamy, they retain the unmistakable characteristics of their uni- 

 cellular ancestors. It must not be supposed that the ancestors of either 

 the multicellular or unicellular organisms any longer exist. The primitive 

 forms from which they may be supposed to have originated have probably 

 long since disappeared in the course of evolution. The Protista of the 

 present day, as well as the individual cells of higher animals and plants, 

 have undoubtedly evolved along different lines and acquired certain charac- 

 teristics which their common ancestors did not possess. Biologists are 

 nevertheless justified in still regarding the portion of cytoplasm with its 

 nucleus as a cell, whether it occurs amongst the Protista or the Metazoa 

 and Metaphyta, in spite of the fact that the cells of each group may now 



