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SCIENCE 



[N. S. Vol. XXXIV. No. 866 



an acid reaction, followed by an alkaline 

 reaction, and conforming in a general way 

 with the digestive processes in higher ani- 

 mals. Few physiologists have attacked the 

 problem of assimilation in protozoa. Ver- 

 worn, however, in his " Biogenhypothese, " 

 has outlined a theoretical conception of 

 the combination of protoplasm molecules 

 with the products of proteid digestion and 

 fcased on the Ehrlich side-chain hj'poth- 

 esis. 



2. Irritability. — Jennings's splendid 

 studies on the behavior of protozoa and 

 lower metazoa have shown that all forms 

 can not be interpreted as simple imits of 

 protoplasm reacting to all external stim- 

 uli by the same simple reflex. A Poteri- 

 odendron, on its simple protoplasmic and 

 filamentous stalk, has but the one reaction, 

 contraction of the stalk, but a Stentor, 

 YorticeUa or Paramecium has not only one 

 but several forms of reaction which are 

 frequently so coordinated as to defy 

 analysis. The reactions, furthermore, vary 

 apparently with the physiological state, 

 or, presumably, with physical and chem- 

 ical states of the protoplasm. Protozoa 

 are thus similar to the lower metazoa and, 

 with them, have been drawn into the field 

 of comparative psychology. 



3. Growth and Reproduction. — Spen- 

 cer's theory of growth and reproduction 

 was soon found to be as unenlightening 

 with protozoa as with higher forms and 

 deeper interpretations have been sought. 

 Few have undertaken to formulate any 

 theory of cell division from protozoa alone, 

 but Hertwig in 1902 advanced a physical 

 theory of growth and division based on 

 his protozoa studies, which has had no 

 little influence. This is now known as the 

 ' ' Kernplasmaspannungstheorie, ' ' or the 

 nucleus-plasma-tension theory. Briefly 

 itated, this theorj^ is based upon the view 

 tAat the ratio of nuclear mass to cyto- 



plasmic mass is constant under certain 

 normal conditions of the cell, and may be 

 expressed by the ratio N/P. If either 

 factor is increased without increase of the 

 other, an " abnormal " condition en- 

 sues. If the P factor increases, as it does 

 with growth, an increasing tension in the 

 cell results in a disturbance of the nuclear 

 conditions and an incitation to regulation 

 by division. If, on the other hand, the nu- 

 cleus plasma ratio is changed to the ad- 

 vantage of the nucleus, ehromidia forma- 

 tion and cell degeneration are the outcome. 

 The bare statement of this theory makes 

 it appear crude and infertile, for it is 

 difficult to see how mass relations can be 

 the cause of growth, division or depres- 

 sion, but if we see in the varying ratio of 

 nucleus to cytoplasm only an index of the 

 chemical interchange going on all the time 

 between the several parts of the cell, and 

 interpret stich variations as effects rather 



Fig. 1. Absence of regeneration in a cut Para- 

 mecium oaudatum. a, normal cell showing plane 

 of cut; 6, anterior truncated fragment; c, division 

 of truncated fragment in original center of cell; 

 d, e, normal and truncated cells resulting from 

 this division; /, division of second truncated cell. 



than as causes, a more plausible explana- 

 tion of the morphological relations of 

 niicleus and cytoplasm is obtained. That 

 excess of nucleus does not cause degenera- 

 tion is shown by a simple experiment. If 

 we ciit Paramecium caud-atum as shown in 

 Fig. 1, a, the cut cell does not regenerate 

 in the majority of cases, but divides in the 

 original central plane of the organism 

 {h, c). As a result of this division one 

 normal (d, anterior) and one abnormal 

 (e, posterior) cell results. The nucleus 



