16 



ZOOLOGY 



Fig. 4. — Diagram to illustrate the reticular theory of 

 protoplasm. (After Dahlgren and Kepiier.) 



billed in the form of an emulsion or froth, the one forming the 

 minute vesicles or bubbles in the froth, the other the ground 

 substance in which the bubbles are embedded (Fig. 3). Accord- 

 ing to another view (reticular theory), one of these substances, 



the less fluid, appears to 

 be arranged in the form 

 of a network of threads, 

 composed of numerous 

 minute rounded granules 

 enclosing the second, 

 more fluid substance in 

 its meshes (Fig. 4). 



To a particle of pro- 

 toplasm, tj'pically con- 

 taining a nucleus in its 

 interior, constituting the 

 entire body of such a 

 simple organism as 

 Amoeba, and forming one 

 of the constituent ele- 

 ments of which a higher plant or animal is made up, the term cell 

 is applied. The word was first employed in reference to the micro- 

 scopic structure of plants, in connection with which it is much more 

 appropriate than in connection with the microscopic structure of 

 animals ; for a plant-cell has, nearly always, a definite, firm, enclos- 

 ing envelope or cM-ivall (Fig. 5, I, c.w) — a structure which is only 

 exceptionally present in the case of animals. In the interior of 

 the cell-protoplasm, or cytoplasm, is a body termed the micleus, 

 similar to the nucleus of Amoeba, and usually of rounded shape, with 

 the appearance of being enclosed in a thin nuclear memlrane 

 (A, nu.vi), perforated by numerous minute apertures. In the 

 nucleus is a single coiled thread, or a network of threads, or one 

 or more rounded clumps, of a substance — chromatin (chr.) — which 

 differs from ordinary protoplasm in having a stronger affinity for 

 most staining agents. A rounded body termed the micleoliis 

 (nu), which usually occurs in the interior of the nucleus, is 

 formed either of a solid mass of chromatin, or of a substance 

 differing somewhat from chromatin in its properties, and less 

 strongly affected by staining agents. When the nucleus divides 

 during the process of division of the cell, its contents, more 

 particularly the chromatin, in many cases go through a remarkable 

 series of changes, to which the term karyoldnesis or mitosis is 

 applied. 



At the time when this mitotic ^ division is about to be 

 initiated, either one or two minute bodies (Fig. 5, A, c) are to be 

 distinguished situated close together in the cytoplasm in the 

 immediate neighbourhood of the nucleus. When only one of 



