No. 494] ZOOLOGICAL PROGRESS 



123 



to conceive of the state of mind of the zoologist of a cen- 

 tury ago so far as his conception of the structure of ani- 

 mals is concerned. To him they consisted of organs com- 

 posed of a variety of unrelated substances, such as 

 muscle, membrane, bone, fat, blood, etc., which consti- 

 tuted the elementary materials of the animal body. 1 1 was 

 therefore a great step forward when Schwann showed in 

 1839 that animals/ like plants, were composed of struc- 

 tural units or cells whose physiological significance, as 

 Briicke afterwards pointed out, entitled them to the name 

 of elementary organisms. Many lower animals, belong- 

 ing to what are now called the Protozoa, proved to be 

 single cells, while the higher animals in all cases were 

 found to be multicellular. The number of cells entering 

 into the formation of the body of one of these higher 

 animals is truly enormous. It is impossible to get re- 

 liable statements for these numbers in any single complex 

 animal, but a recent careful estimate of the number of 

 ganglion cells in the human cerebral cortex places the 

 total at 9,200,000. As this enormous number of cells 

 would occupy less than a cubic inch of space, one can 

 form some rude conception of the prodigious number in 

 the whole human body. 



It is a remarkable fact that almost all cells, whether 

 they are whole single animals or only parts of animals, 

 are of small size. There seems to be something about the 

 organization of a cell which ordinarily prevents it from 

 enlarging much beyond microscopic proportions. The 

 essential parts of a cell are its nucleus and the surrounding 

 cytoplasm, and the continued activity of the cytoplasm is 

 known to be dependent upon the presence and integrity 

 of the nucleus. Such being the case it would seem as 

 though the nucleus could administer to only a limited 

 volume of cytoplasm and thus restricts the size of the cell. 

 Apparently, however, this relation is one of volume and 

 not of distance from the nucleus, for the cytoplasm of a 

 ganglion cell may be drawn out into a most delicate nerve- 

 fiber process that may reach half the length of the human 

 body. 



