10 COMPARATIVE ANATOMY. 



spaces occupied with homogeneous liquid. Three concen- 

 tric spheres can commonly be detected : the first (nucleolus) 

 placed within the second (nucleus); the third (cell wall) en. 

 closing the other two. Homogeneous granules appear within 

 the liquid, and filaments (cilia) may be appended to the cell 

 wall. 



FORMS OF CELLS are round, oblong, elongate, cylindrical, 

 compressed, polygonal, and stellate. Surrounding conditions 

 and position determine shape. 



PROPERTIES OF CELLS. In addition to the nutritive pro- 

 cesses common to all organisms, may be mentioned fissura- 

 tion and motility. 



Relations of cells to histogenesis lead to consideration of 

 origin and significance of cells. 



1st Theory. Cells originate spontaneously in a homoge- 

 neous fluid, in the form of granules (nucleoli). Around these 

 granules are subsequently developed others assuming the 

 form of the remaining constituents of the cell, to be recog- 

 nized as the nucleus and cell wall. (Schleiden and Schwann.) 



2d Theory. Omni ceUulce e cellula. (Virchow.) 



3d Theory (generally known as the germinal theory). The 

 terminology of the cell is discarded, and the phrases "ger- 

 minal matter" and " formed matter" substituted. The former 

 is a structureless plastic substance everywhere pervading 

 organized bodies, to which the formed matter stands in the 

 relation of an evolved substance. (Beale.) 



ORGANISM. 



All organisms are sexual beings, and are, for the most 

 part, composed of cells. 



Many inferior organisms assume the form of cells, as 

 Amoeba* and Bodo. The ovum, apparently a cell, is in re- 

 ality an undeveloped organism. The difference between a 



* Amoeba, one of the lowest of Rhizopoda, composed of sarcode, with 

 an outer structureless diaphanous integument, and containing an interior 

 circular contractile vesicle. Measurement, from ^ to 2 ^ 5 " long. Is found 

 in stagnant water. 



