142 The Microscope. 



:spoken of the primordial utricle and the nucleus, he adds : 

 " The remaining part of the cell is more or less filled by an opa- 

 line, viscous, white and granular substance, which I call proto- 

 plasm." 



Thus, according to von Mohl, in a complete cell, with mem- 

 brane and nucleus, the name protoplasm is reserved to the hya- 

 line and viscous part, which remains between the two in its 

 primitive state. 



Von Mohl, by his interesting researches had realized an im- 

 mense progress in the science of Cytology ; yet one mistake he 

 made, and that is, he confined himself to the study of the higher 

 plants. 



Thus while Von Mohl confined himself to the plant cell, other 

 investigators applied themselves to the study of the animal cell, 

 :and particularly to the Protozoans, and arrived at conclusions 

 .somewhat different from those of Von Mohl. Schwann, who 

 had regarded the membrane as an essential part of both the 

 animal and vegetable cell, now abandons his position, and con- 

 fesses its absence in the white blood-corpuscles ; and, the study 

 ■of the zoospores of algse and fungi and of the Myxomycetes, 

 *tc., more even than the study of the higher animals and plants, 

 rsoon convinced most investigators that the views of Von Mohl 

 required great modification. 



Leydig, in 1856, was the first to abandon the definition of the 

 •cell which till his time had obtained. " The contents of the 

 <;ell," he says, " are of greater importance than the membrane ; 

 for there is the seat of irritability and contractility." About the 

 same time (1861), Brucke doubted whether the nucleus is an 

 essential part of the cell, since among crj'^ptogams many cells 

 are found devoid of a nucleus. Schwann (1854), in the study 

 of the Amoeba porreda, had already noticed the absence of a 

 aiucleus. Soon many more examples of the absence of a nu- 

 cleus in cells multiplied to such an extent that Haeckel deemed 

 it necessary to establish a whole group of animals, the Moners 

 or cytodeS; characterized by the absence of the nucleus. At this 

 time, therefore, we find many scientists who regard the cell as 

 " a simple globule of protoplasm," which even by some is re- 

 garded as a mere speck of albumen^. 



2 The reader may peruse with great profit the first part of the article on Physi- 

 ology in the Cyclopaedia Britannica. 



