CELL THEOEY 5 



highly developed and most complex in structure possess definite nuclei, 

 and the small particle of cytoplasm with its included nucleus of which 

 the body of each is composed is regarded by most biologists as a cell on 

 account of its resemblance to the cells of higher animals and plants. 



The term "cell" was first introduced for the cellulose capsule or wall 

 which encloses the portions of cytoplasm of which the higher plants are 

 built up. It was later realized that the wall itself was merely a sup- 

 porting structure, and that the cytoplasm within it was in reality the 

 living material. Accordingly, the term "cell" was then applied, not to 

 the cell wall, but to its cytoplasmic contents. The latter consists typically 

 of a small mass of cytoplasm containing a single nucleus. When it was 

 discovered that the tissues of higher animals were also built up of similar 

 elements or units, the term "cell" was applied to them also. It soon 

 became evident that, in the case of many microscopic organisms, the entire 

 body consisted of a similar mass of cytoplasm containing a nucleus, and 

 the resemblance of these to the cells of higher animals and plants gave 

 rise to the view that these organisms were single cells, and the distinction 

 between unicellular and multicellular animals was drawn. This con- 

 ception, which was first clearly expounded by Schwann (1839), has been 

 generally accepted, though Dobell (1911) considers it erroneous. He 

 believes that an amoeba, for instance, is as much an entire organism as 

 one of the higher animals, and that though the latter may be regarded 

 as being multicellular, as a result of the division of its cytoplasm and 

 nucleus into cells, the former should be considered as a non-cellular 

 organism, and not a unicellular one, since it corresponds, not to any 

 single cell, but to all the cells which compose the body of the multi- 

 cellular organism. 



When it is realized that amongst the numerous cells which compose the 

 body of one of these higher animals there are many wandering cells, such 

 as macrophages, which behave in all essential respects like amoebae, in that 

 they form pseudopodia, ingest solid proteid material of various kinds, and 

 multiply by fission, it is difficult to resist the conviction that such a cell has 

 a definite claim to be regarded as an individual organism like an amoeba 

 itself. Furthermore, it has been clearly demonstrated that very minute 

 portions of the tissues, consisting of groups of cells, or even single cells of 

 higher animals, can be artificially cultivated, and that they will live and 

 multiply indefinitely provided they are given a continuous supply of suit- 

 able nutriment. From these culture experiments it seems clear that the 

 cell, which forms but a part of the entire multicellular animal, is capable of 

 nourishing itself and reproducing as a single organism. Another illustration 

 of the power of independent existence and multiplication of isolated cells 

 of multicellular animals is seen in malignant disease. In this condition 



