THE STUDY OF ZOOLOGY 17 



heredity. A detailed account of cell division and the behavior 

 of the chromosomes is reserved for a later chapter. A nucleolus 

 is also usually present in a nucleus. It is as a rule rounded in 

 shape and stains differently from the chromatin. Its function is 

 unknown. The remainder of the nucleus is composed of karyo- 

 ly?nph which serves as a matrix in which the chromatin and 

 nucleolus are suspended. This ground substance is semifluid in 

 character. 



In the cytoplasm the principal formed elements common to all 

 cells are the mitochondria and the Golgi material (Figs. 3 and 4). 

 The mitochondria are minute objects, usually 

 rod-shaped, that occur in large numbers in the 

 cytoplasm. The Golgi material is usually in 

 the form of a closed network of fine threads 

 or canals which may extend throughout the 

 cytoplasm or be confined to a more restricted 

 area. Both mitochondria and the Golgi 

 material can be seen in living cells. Materi- 

 als such as fat, oil globules, yolk, glycogen, 

 etc., occur in varying amounts as inclusions in ganglion cell of the 

 the cytoplasm of cells. The ground substance rabbit s . howi "f Golgi 



J l ° material in the form 



of the cytoplasm in which these structures and of a network in the 

 materials are suspended or dispersed is opti- J^^J "entra^ob- 

 cally homogeneous. Cilia (short hairlike proc- j ec t is the nucleus. 

 esses), flagella (long whip-shaped structures), (After Golgi.) 

 and membranellae (delicate fin-shaped membranes) may develop 

 as cytoplasmic structures at the surface of some cells. In general 

 there is a greater degree of variability in differentiation and form 

 in the cytoplasm than in the nucleus. 



The complex nature and orderliness of the chemical reactions 

 that take place in the living cell seem to call for some sort of 

 structural protein framework within the cell by means of which 

 the cytoplasm is subdivided and supported. Such a structural 

 framework has been demonstrated in the following manner: 

 Fresh cells such as those of the liver of the rabbit, are frozen in 

 liquid air at — 30°C. and dried under partial vacuum for 12 

 hours, and then ground to a flour in a mortar. If the ground up 

 powder is then treated with salt solutions all of the soluble protein 

 in the form of globulins can be extracted. The solid residue from 

 which the extraction is made when examined under the micro- 



