20 PHYSIOLOGY 



from the existence of a solid cell wall, the cell presents a certain degree 

 of rigidity and resistance to deforming stress. 



ULTRAMICROSCOPIC STRUCTURE OF PROTOPLASM. Since the study of 

 the behaviour of the cell shows that it must possess a much more complex structure 

 or organisation than that which is revealed by the microscope, one, that is to say, 

 which permits of the spatial differentiation of the different chemical processes that 

 may occur at one and the same time in the protoplasm, many theories have been put 

 forward of an ultramicroscopic cell structure. Though Spencer in 1864 spoke of 

 physiological units out of which protoplasm could be regarded as made up, and Darwin 

 (1868) conceived ultramicroscopic particles gemmules which might be discharged 

 from every cell in the body and, passing into the reproductive organs, serve as the 

 material basis of heredity, the first elaborate conception of such a structure was worked 

 out by Nageli (1884). According to Nageli all organised structures are made up of 

 micellae, minute particles arranged in definite order and surrounded with water. For 

 growth to take place it was necessary that the system should be in a condition of 

 ' turgor,' which was determined by the amount of water between the micellae. These 

 micellae arose in every case from the division of pre-existing micellae, and the vital 

 properties of the protoplasm were to be regarded as the sum of the changes taking 

 place in the individual micellae. Similar conceptions have been put forward by numerous 

 other observers, each of whom has applied a different name to the elementary living 

 particle, such as ' pangene,' ' plasome,' * biophor,' ' biogen-molecule,' and many others. 

 The resemblance of these theories to that of Altmann is obvious, though the latter 

 regarded the elementary particle as in many cases of microscopic size and capable of 

 demonstration by appropriate methods of staining. That the cell possesses organs 

 of smaller dimensions than itself, which may give rise to like organs by division, is 

 shown by Schimper's observations on the plastids of plant cells. These apparently 

 are not formed by a process of differentiation of the protoplasm, but are continuous 

 from one generation to another and are reproduced by division. There is no doubt, 

 however, that most of the granules to be observed in the cytoplasm are not of this 

 character, but are elaborated by the general cytoplasm out of the foodstuffs which are 

 supplied to it ; and though conceptions such as those of De Vries and Verworn are 

 often of value as a means of describing certain phenomena in the life of the cell and have 

 played a great part in the description of the phenomena of heredity, they cannot be 

 regarded as having any serious justification in fact. At the present time our know- 

 ledge of the properties of the colloidal and capillary systems, which must play so great 

 a part in the organisation and reactions of living protoplasm, is much too meagre to 

 justify weight being laid on any theory of the ultramicroscopic structure of protoplasm 

 that can at present be put forward. 



One question which has been much discussed relates to the physical 

 condition of protoplasm. Is it to be regarded as a viscous fluid or as a soft 

 solid ? The perfect potential mobility of the protoplasm of many cells, as 

 instanced by the flow of a substance of an amoeba into its pseudopodia, or the 

 occurrence of rapid streaming movements in the threads of protoplasm 

 found in many plants, e.g. the root hairs of tradescantia, indicates a fluid 

 character for the protoplasm. Against such a character has been urged the 

 fact that in protoplasm we may have shape, organisation, and power of 

 resistance to deformation qualities which are generally associated with the 

 possession of solidity. It must be remembered, however, that the absence of 

 resistance to deformation, which is characteristic of a liquid, applies only to 

 the internal molecules, and that the surface of any liquid is in a condition 

 of tension which not only limits deformation, but presents considerable 

 resistance to any enlargement of the surface. Small water animals take 



