2 TEXT-BOOK OF EMBRYOLOGY. 



only for some resting cells, is always present in cells which are undergoing 

 mitotic division. 



In addition to the net knots are the true nucleoli or plasmosomes. These are 

 spheroidal bodies which lie free in the meshes of the nuclear reticulum. They 

 vary in number in different cells and sometimes in the same cell in different 

 conditions of activity. They stain intensely with basic dyes. The function 

 of the nucleolus is not known. It has been regarded by some as material in 

 process of constructive metabolism, by others as a waste product. 



The nucleus is typically spherical. Its shape may or may not be modified 

 by the shape of the cell body. Nuclei may assume very irregular shapes, as in 

 polymorphonuclear leucocytes, or they may be lobulated, as in some of the 



Cell membrane 



Metaplasm 1 



granules 



Karyosome or ] 

 net knot J 



Hyaloplasm 

 Spongioplasm 



Linin network 

 Nucleoplasm 



Aster (attraction-sphere) 

 Centriole 



Plastids (metaplasm) 



Chromatin network 

 Nuclear membrane 



Nucleolu 



Vacuole 



Fig. i. — Diagram of a typical cell. Bailey. 



large cells of bone marrow; or a cell may have a number of nuclei. The shape 

 of the nucleus may vary considerably within comparatively short periods of time. 

 Such nuclei have been described as having amoeboid movement. The size 

 of the nucleus also appears to be independent of the size of the cell body, some 

 large cells having small nuclei, while some small cells are almost completely 

 filled by their nuclei. The nucleus tends to lie near the center of the cell, yet 

 may be eccentric to any degree and appears to be suspended in the cytoplasm 

 in such a way that its location within the cell may change. In some of the lowest 

 forms no true nuclear structure exists, scattered granules of chromatin consti- 

 tuting the rudimentary nucleus, generally called a diffuse nucleus. 



As the nucleus is an essential element in all reproduction, it follows that all 

 cells have been nucleated at some time in their developmental history, and that 

 the adult nonnucleated condition of some cells (e.g., respiratory epithelium) 

 is indicative of their having passed beyond the age of reproductive power. If 

 the nucleus be removed from a living cell, the cytoplasm does not necessarily 



