STRUCTURE OF CELL AND NUCLEUS 9 



optical section, has the appearance of a network, and is known as the 

 linin network, of which the nuclear membrane may be regarded as a special 

 development. Upon this network, and on the nuclear membrane in the 

 form of granules or larger masses, is arranged another substance, the 

 chromatin, which has a strong affinity for certain stains. It is generally 

 regarded as the most important constituent of the nucleus, and this is 

 borne out by the fact that nuclear division takes place by an elaborate 

 process known as 7nitosis, which results in an equal sharing of the chromatin 

 between the daughter nuclei. In the nucleus of the Metazoan cell there 

 is usually present a conspicuous body known as the nucleolus. It is devoid 

 of chromatin, and when nuclear division takes place it passes to one of 

 the daughter nuclei, the other daughter nucleus forming a new nucleolus. 

 A very similar body exists in the nuclei of certain Protozoa (Opalina), 

 and it passes to one of the daughter nuclei when division takes place. In 

 other Protozoa, as, for instance, in Karyolysus and Hepatozoon, a similarly 

 achromatic body divides at nuclear division, each daughter nucleus re- 

 ceiving half (Fig. 35). When such a body occupies a central position in 

 a Protozoan nucleus it is known as a haryosome, and it has been generally 

 assumed that it is composed largely of chromatin. It is becoming in- 

 creasingly evident, however, that the karyosome may be actually devoid 

 of chromatin, and the supposition that in certain nuclei the entire chro- 

 matin may be concentrated in the karyosome is a very doubtful one. The 

 nucleus is often regarded as consisting of two substances — the achromatic 

 and the chromatic material. The achromatic material, including the 

 nuclear membrane, linin network, nuclear sap, and other bodies (karyo- 

 some, nucleolus) which are sometimes present, undoubtedly comprise 

 several distinct substances, some of which, at any rate, are able to give 

 rise to chromatin, for the quantity of chromatin in the nucleus varies from 

 time to time, and increases with its growth. Ahother important constituent 

 of the cell, which as a rule only becomes visible during nuclear division, 

 is the centrosome (Fig. 1, A). It is commonly present in the cells of 

 Metazoa, but it is not so frequently seen in the Protozoan cell. Repro- 

 duction of a cell by binary fission or multiple segmentation is always 

 preceded by division of the centrosome, if one is present, followed by 

 division of the nucleus, which in most cases takes place by mitosis. It 

 is during nuclear division that the nature of many of the constituents of 

 the nucleus first comes to light, and for this reason it will be necessary to 

 consider mitosis, as it occurs typically in the Metazoan cell. During 

 mitosis there are formed, mainly out of the chromatin, certain bodies 

 known as chromosomes, which are constant in number for each species of 

 animal, the same number appearing at each succeeding nuclear division. 

 There is some evidence that in the resting, or more accurately the non- 



