358 CELL-CHEMISTRY AXD CE/.f-PL/YSLOLOCY 



as cilia, flagelki, and pseudopodia,^ the centrosomes in ciliated cells 

 and spermatozoa, and in the swarm-spores of Noctiluca, is, however, a 

 most strikini;- fact, and is one of the strongest indirect arguments in 

 favour of the general theory of fibrillar contractility in mitosis. 



D. Summary and Conclusion 



The facts reviewed in the foregoing pages converge to the conclu- 

 sion that the differentiation of the cell-substance into nucleus and V/ 

 cytoplasm is the expression of a fundamental physiological divisiony^ 

 of labour in the cell. Experiments upon unicellular forms demonstrate 

 that, in the entire absence of a nucleus, protoplasm is able for a 

 considerable time to liberate energy and to manifest coordinated 

 activities dependent on destructive metabolism. There is here sub- 

 stantial ground for the view that the cytoplasm is the principal 

 seat of these activities in the normal cell. On the other hand, 

 there is strong cumulative evidence that the nucleus is intimately 

 concerned in the constructive or synthetic processes, whether chemical 

 or morphological. 



That the nucleus has such a significance in synthetic metabolism 

 is proved by the fact that digestion and absorption of food and 

 growth soon cease with its removal from the cytoplasm, while destruc- 

 tive metabolism may long continue as manifested by the phenomena 

 of irritability and contractility. It is indicated by the position and 

 movements of the nucleus in relation to the food-supply and to the 

 formation of specific cytoplasmic products. It harmonizes with the 

 fact, now universally admitted, that active exchanges_of^material 

 go_j3n between nucleus and cytoplasm. The periodic changes of 

 staining-capacity undergone by the chromatin during the cycle of cell- 

 life, taken in connection with the researches of physiological chemists 

 on the chemical composition and staining-reactions of the nuclein 

 series, indicate that the phosphorus-rich substance known as iiuclcimc 

 acid plays a leading part in the constructive process. During the 

 vegetative phases of the cell this substance is combined with a large 

 amount of the albumin radicles histon, protamin, and related sub- 

 stances, and probably in part with albumin itself, to form nuclein. 

 During the mitotic or reproductive processes this combination appears 

 to be dissolved, the albuminous elements being in large part split 

 off, leaving the substance of the chromosomes with a high percentage 

 of nucleinic acid, as is shown by direct analysis of the sperm-nucleus 

 and is indicated by the staining-reactions of the chromosomes. There 

 is, therefore, considerable ground for the hypothesis that in a chemi- 

 cal sense this substance is the most essential nuclear element handed 



1 Cf. pp. 92, 102, on the central granule of the Heliozoa. 



