4 GERM-CELL CYCLE IN ANIMALS 



wise have taken place in the Cell Theory; we no 

 longer consider cells as isolated units and the multi- 

 cellular animal as equivalent to the sum of its con- 

 stituent cells, but recognize the influence of the cells 

 upon one another, thus reaching the conclusion that 

 the metazoon represents the sum of the individual 

 cells plus the results of cellular interaction. 



Cells vary considerably in size, ranging from those 

 we call Bacteria, which may be no more than ^^^-^-^ 

 of an inch in length, to certain egg cells which are 

 several inches long; the latter, however, owe their 

 enormous size to the accumulation of nutritive sub- 

 stances within them. An average cell measures 

 about 2^Vo ^^ ^^ i^^ch in diameter. Cells vary in 

 shape as well as in size; egg cells are frequently 

 spherical, but most cells are not, since they are sur- 

 rounded by other cells which press against them. 

 A diagram of a typical cell is shown in Fig. 1. 



Authorities are not agreed as to the structure of 

 protoplasm; to some it appears, as shown in Fig. 1, 

 to consist of a network of denser fibers called spon- 

 gioplasm (s) traversing a more liquid ground 

 substance, the hyaloplasm. Others consider proto- 

 plasm to be alveolar in structure, thus resembling 

 an emulsion, whereas another group of zoologists 

 maintain that while protoplasm may appear to be 

 fibrillar or alveolar, its essential basis consists of 

 multitudes of minute granules. Wilson's view is 

 the one usually adopted at the present time; that 

 is, the protoplasm of the same cell may pass suc- 

 cessively "through homogeneous, alveolar, and 



