METHODS OF PHYSIOLOGICAL RESEARCH 53 



such as the cells of the central nervous system, glands, and 

 muscles. 



We need not be embarrassed in the employment of experi- 

 mental physiological methods upon the cell, for, with the over- 

 whelming variety of forms in existence, more than one can always 

 be found that are equally fitted for the purpose, and upon which 

 widely different, special methods may be advantageously used. 



To begin with the simplest method, simple microscopic olservatimi 

 may be employed very conveniently with the free-living cell, and 

 under certain circumstances with the tissue-cell also. Observation 

 alone has led to a fair knowledge of the visible vital phenomena 

 of the cell, and has been used in the detailed investigation of 

 some of them. Among the most prominent acquisitions by this 

 simple method may be mentioned the extremely valuable facts 

 concerning the more detailed phenomena of fertilisation, segmenta- 

 tion, and reproduction which Flemming, Btitschli, van Beneden, 

 the brothers Hertwig, Strasburger, Boveri, Heidenhain, and many 

 others have discovered in recent years, partly on living cells, and 

 partly on cells that have been preserved in certain stages. 



Vivisection-operations upon the cell may also be performed under 

 the microscope to the same extent and with greater systematic 

 exactness than they are performed macroscopically upon higher 

 animals. Several investigators, such as Gruber, Balbiani, Hofer, 

 and others have already employed this operative method with 

 great success, and a number of researches have shown how fruitful 

 it is for the treatment of general physiological problems. By this 

 method also Roux, Chabry, the brothers Hertwig, Driesch, and 

 others have carried out their striking experimental investigations 

 upon the development of animals. 



Further, a great variety of studies can be made upon the effects 

 of different kinds of stimuli upon the vital phenomena of the cell 

 in its various forms ; in this field a comprehensive mass of facts 

 has already been accumulated. A large number of researches 

 upon unicellular organisms have shown that the reactions that 

 appear in the cell upon the employment of chemical, mechanical, 

 thermal, photic, and galvanic stimuli, are of the greatest import- 

 ance in a knowledge of vital phenomena. By these researches it 

 has been made possible in recent years to recognise more and more 

 clearly the general laws of excitation and depression of vital pro- 

 cesses and their results, and also to approach nearer an under- 

 standing of the phenomena of inhibition, which hitherto have 

 been so obscure. 



Finally, vital phenomena in the cell can be approached chemi- 

 cally, by both macrochemical and microchemical methods. Large 

 masses of unicellular organisms, such as yeast-cells, leucocytes, and 

 spermatozoa, and no less combinations of cells, such as the tissues, 

 form excellent objects for macrochemical investigation. We are 



