INTRODUCTION 7 



dria and other cell components, but it Is hoped that improved tech- 

 nique in fixation and staining, together with the development of 

 tissue culture studies and the micro-manipulation of living cells, 

 will reveal the functions of real structures and indicate which 

 granules are only artifacts. 



There are other scattered bodies in the cytoplasm, not readily 

 seen but "haloed" under strong illumination, which may be the 

 ultimate units from which chondriosomes and Golgi-bodies are 

 aggregated. 



Nucleus. — The nucleus is visible in living cells. Some cells 

 contain several nuclei, as in the giant cells of bone marrow, and 

 many others are found to contain two types of nuclei, certain 

 protozoa, In fact, having both a nutritive and a reproductive 

 nucleus. The nucleus is essential to the life of the cell, and is related 

 to metabolism and the secretory phenomena. 



The nuclear monbrane, tough and elastic. Is found In the majority 

 of animal cells, but is absent in certain protozoa. It disappears 

 during mitosis (indirect cell division, page 500). 



The nucleo-reticulum consists of chromatin, which Is stained by 

 basic dyes. Chromatin may appear in the form of fine granules, 

 larger granules, or in a network, with net-knots of massed granules. 

 (See Fig. i.) 



Chromatin granules {chromomereSy chromioles) are suspended on 

 a reticulum of linin, which is delicate In structure and stained by 

 acid dyes. The granules aggregate to form chromosomes,'* which 

 are constant In number for the cells of the same species. Appar- 

 ently the granules are not dissociated, but usually prepared to 

 reassemble. The chromosomes are carriers of hereditary /«f/orj or 

 "genes," although we are not certain that they are the sole means of 

 transmission from one generation to the next. (See p. 538.) 



Nucleolus. — The nucleolus or plasmosome Is a dense body, 

 spherical in shape, and chemically different from chromatin since 

 it Is stained with acid dyes. During cell division, the nucleolus 

 usually disappears. Sometimes there are several nucleoli, which 

 may play a part in the metabolism of the cell. 



Structure of Protoplasm. — The fact that protoplasm differs 

 under various physiological conditions has given rise to several 



*" Chromosomes are individual chromatic elements which appear definitely in 

 the nucleus at the end of the prophase stage and which act as unit structures during 

 mitosis." E. Carothers. See page 500 for a discussion of mitosis. 



