HANDBOOK OF PHYSIOLOGY. 



The phenomena of life then are exhibited in cells, whether existing 

 alone or developed into the organs and tissues of animals and plants. 

 It must be at once evident that a correct knowledge of the nature and 

 activities of the cell forms the very foundation of physiology; cells 

 being, in fact, physiological no less than morphological units. 



The prime importance of the cell as an element of structure was first 

 established by the researches of the botanist Schleiden, and his conclu- 

 sions, drawn from the study of vegetable histology, were at once ex- 

 tended by Theodor Schwann to the animal 

 kingdom. The earlier observers defined a cell 

 space ccm- as a more or less spherical body limited by a 

 SjSid g membrane, and containing a smaller body 



.Protoplasm. 

 .Nucleus. 



Cell-wall. 



Fig. l.-Vegetable cells. 



Nucleus or germinal 

 . vesicle. 



Nucleolus or germi- 

 nal spot. 



Space left by retrac- 

 tion of yelk. 



.Yelk or vilellus. 



.Vitelline membrane. 



Fig. 2. Semidiagrammatic representation of a human 

 ovum, showing the parts of an animal cell. (Cadiat.) 



termed a nucleus, which in its turn incloses one or more still smaller 

 bodies or nucleoli. Such a definition applied admirably to most vege- 

 table cells, but the more extended investigation of animal tissues soon 

 showed that in many cases no limiting membrane or cell-wall could be 

 demonstrated. 



The presence or absence of a cell- wall, therefore, was now regarded 

 as quite a secondary matter, while at the same time the cell-substance 

 came gradually to be recognized as of primary importance. Many of 

 the lower forms of animal life, e.g., the Ehizopoda, were found to con- 

 sist almost entirely of matter very similar in appearance and chemical 

 composition to the cell-substance of higher forms; and this from its 

 chemical resemblance to flesh was termed Sarcode by Dujardin. When 

 recognized in vegetable cells it was called Protoplasm by Mulder, while 

 Remak applied the same name to the substance of animal cells. As the 

 presumed formative matter in animal tissues it was termed Blastema, 

 and in the belief that, wherever found, it alone of all substances has to 

 do with generation and nutrition, Beale has named it Germinal matter 

 or Bioplasm. Of these terms the one most in vogue at the present day, 

 as we have already said, is Protoplasm, and inasmuch as all life, both in 

 the animal and vegetable kingdoms, is associated with protoplasm, we 



