22 GENERAL BIOLOGY 



a structure that appears only at certain periods of 

 the cell's activity and is either invisible or non- 



FIG. 5. Various kinds of cells : A, female germ-cell, ovum of the 

 cat ; B, male germ-cell, spermatozoon of a snake, Coluber ; C, ciliated 

 epithelium from the digestive tract of a mollusk, Cyclas ; D, cartilage of 

 a squid ; E, striated muscle fiber from an insect larva, Corydalis 

 cornutus; F, a nerve cell from the cerebellum of man. (Dahlgren 

 and Kepner.) 



existent throughout the greater part of cell life. 

 (See under Cell Division, Chapter IV.) 

 The nucleus (sometimes called karyoplasm in 



