Guilliermond - Atkinson — 2 — Cytoplasm 



A little later, Cohn (1850), Thuret (1850) and Pringsheim 

 (1854) perceived that the zoospores of the algae lack a wall and 

 are made up exclusively of protoplasm. Max Schultze and DE 

 Bary (1859) finally established the fact, once for all, that the 

 protoplasm of plants is the essential substance of cells and corre- 

 sponds to the sarcode of Dujardin. Leydig (1857) defined the cell 

 as "a mass of protoplasm furnished with a nucleus". Max 

 Schultze (1861) defined it as "a mass or lump of protoplasm 

 endowed with vital properties". 



While these conceptions were being established, the works of 

 VON MoHL, Meyen and Nageli were proving the inexactitude of 

 Schleiden's theory of cell origin and showing that cells multiply 

 by division. Thus the word cell (French cellule, from the Latin 

 cellula, little room) came to mean the contents of the cellular cav- 

 ity, a significance quite different from that given it by HOOKE, who 

 observed only the walls. 



From then on, the conception of the cell was enlarged upon 

 but the knowledge of its structure, making only slow progress, was 

 to remain obscure for a long time. The early cytologists observed 

 only living cells. This presents serious diflflculties, for, with the 

 exception of unicellular organisms, observation of living material 

 can be carried out only after tearing or sectioning tissue, operations 

 which risk injuring the cells. Cells examined in a medium not their 

 own — water, for instance — may, during observation, undergo 

 serious alterations. Lastly, observation of living material never 

 allows the study of cell structure to be pushed sufficiently far, be- 

 cause, except for cases which are unusually favorable, the 

 different elements which constitute the cell show too small differ- 

 ences of refractivity for it to be possible to distinguish one from 

 the other with clearness, and this becomes, moreover, well nigh 

 impossible in embryonic tissue in which the cells are very small. 



The introduction of the paraffin method on objects pre- 

 viously "fixed" has greatly facilitated the work of cytologists. 

 This method consists in fixing, i.e., coagulating, the cells by means 

 of various chemical reagents, then embedding the tissues in paraflSn, 

 cutting them in thin sections with the aid of a microtome and 

 finally staining them. Several stains may then be applied to the 

 sections, which fix this constituent or that, according to its affinity 

 for the stain, and superb preparations may be obtained and made 

 permanent in Canada balsam. But this method, convenient as it 

 is, has, nevertheless, the serious difficulty of reducing cytologists 

 to the study of dead cells only. Fixation, i.e., coagulation, of proto- 

 plasm, modifies the structure of the cell and exposes cytologists to 

 serious errors in interpretation. Finally, this method does not 

 allow the physical character or biological properties of the cyto- 

 plasm to be studied, although they are very important, and cytology 

 is reduced to pure cellular morphology. The use of the paraffin 

 method has led to the striking discovery of karyokinesis and has 

 rendered very great service in the study of the nucleus and, in par- 



