48 



ESSENTIALS OF ZOOLOGY 



A cell consists of a mass of jellylike cytoplasm surrounding a 

 nucleus. The outer surface of the cytoplasm is modified, the proto- 

 plasm having more density here to form the plasma membrane, or 

 cell membrane, which is the outer covering of the animal cell. It 

 is living and semipermeable. 



The cytoplasm usually includes the larger part of the substance of 

 the cell. It may be subdivided into the more nearly clear, structure- 

 less fluid, hyaloplasm, and the interspersed fibrillar substance known 

 as spongioplasm. Within the cytoplasm, lying near the nucleus, in 

 most animal cells is the centrosome. Its substance is kno^vn as kino- 

 plasm and is made up of two parts, the larger centrosphere, enclosing 



.CELL MEMBRANE 

 PLASMA MEMBRANE 

 CENTROSOME 

 CENTRIOLE 



NUCLEAR MEMBRANE 

 NUCLEOLUS 



CHROMATIN RETICULUM 



-NUCLEAR SAP 



PLASTIO 



VACUOLE 

 ENDOPLASM 



ECTOPLASM 



CHONDRIOSOME 



) NUCLEUS 



Fig. 11. — Diagram, showing a typical celL (From Parker and Clarke, Introduction 

 to Animal Biology, The C. V. Mosby Company.) 



a (two if divided) cent Hole. Vacuoles are often present as small 

 cavities filled with water, gases, or oils. Scattered through the cyto- 

 plasm also are numerous rod-shaped bodies known as mitochondria. 

 Threadlike golgi elements or apparatus may be observed in the cyto- 

 plasm, particularly near the nucleus. Secretions produced in the cell 

 may be stored as granules in the cytoplasm, also certain inclusions 

 may be seen here. 



The nucleus, which is usually round and centrally located, is sur- 

 rounded by the cytoplasm and separated from it by the nuclear 



