60 TEXTBOOK 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. This 

 membrane is living and semipermeable. In some types of cells two 

 separate membranes may be distinguished. In plant cells the plasma 

 membrane is covered by a cellulose cell wall. 



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 known as kino- 

 plasm and is made up of two parts, the larger ce7itrosphere, enclosing 

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



lis I 



h ' 



Fig. 21. — A camera lucida drawing, showing- the details which appear on the 

 upper 5?urface of a fully developed salivary gland chromosome (large vesicle type) 

 from Simulium fly larvae. The longitudinal threadlike bands are called chromone- 

 mata, and these consist of a linear series of granules, the chromomeres, which 

 have a specific arrangement of grouping. lA is a semidiagrammatic representation 

 of the types of chromomeres and the ways in which they are connected. At a in 

 the main figure there are two rows of dotlike chromomeres which tend to associate 

 in pairs. The band labeled h is composed of 15 or 16 vesiculated chromomeres 

 closely pressed together, c to li are other groupings of chromomeres along the 

 chromonemata of the chromosome. (From Painter and Griffen : Chromosomes of 

 Simulium, Genetics 22: 616, 1937.) 



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 gi-anules 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 

 membrane. This membrane, like the plasma membrane, consists of 

 a part of the protoplasm whose density is somewhat greater than the 

 adjacent portions. The protoplasm which constitutes the nucleus is 

 usually known as karyoplasm. The more nearly fluid, transparent 



