246 THE PROTOZOA 



An ordinary cell in Metazoa and the higher plants consists of pro- 

 toplasm, which is typically differentiated into cell-body or cytoplasm, 

 and nucleus. These areas of protoplasm differ considerably in their 

 chemical composition, the former being rich in proteid, of which 

 albumins play the most important part, and poor in phosphorus ; the 

 latter, on the other hand, being rich in phosphorus, which is bound up 

 in a substance called nuclein, and poor in albumins. 



Both nucleus and cytoplasm, as seen under the high power of a 

 microscope, have a complicated structure. In both there appears to 

 be a general ground substance, the cytolymph in the cell-body, and 

 the karyolymph in the nucleus. Throughout this ground substance, 

 in both the cell-body and the nucleus, extends an alveolar meshwork, 

 the intra-nuclear portion being known as the linin reticulum (Fig. 

 133). The cytoplasm frequently contains other structures, such as 

 plastids, vacuoles, metaplasmic bodies, crystals of various kinds, etc., 

 which may or may not be permanent in the cell. A more important 

 structure is the centrosome, which is usually present in the Metazoa 

 and which has been generally considered as playing a leading role 

 in cell-division. This body, which is extremely minute, is, as a rule, 

 surrounded by a granular area of cytoplasm known as the attraction- 

 sphere > centrosphere, or simply as the sphere. The nucleus, in addition 

 to the linin reticulum and the karyolymph, contains chromatin, a sub- 

 stance which, more than anything else, distinguishes the nucleus 

 from the cytoplasm. As a rule, chromatin appears in the form of 

 small granules distributed through the linin network. These granules 

 in the resting nucleus may be close enough together to give the ap- 

 pearance of a chromatin reticulum, while during division of the nucleus 

 they become fused, forming structures known as chromosomes. The 

 latter, save in certain stages in the formation of the sex-cells, are of 

 definite number and appearance for the same species. The chromatin 

 network is frequently thickened to form relatively large masses of 

 chromatin known as net-knots, or, together with the plasmosomes, as 

 nucleoli. The plasmosomes, or true nucleoli, are comparatively large, 

 spherical masses suspended in the karyolymph and are of unknown 

 function. The nucleus, finally, is inclosed within a definite membrane 

 which usually disappears during mitosis. 



Although the main features of indirect nuclear division, or mitosis* 

 in animals and plants are so similar that a common type of mitotic or 

 division-figure can be described, there are, nevertheless, confusing 

 variations and differences, especially in the so-called "achromatic" por- 

 tions of the mitotic figure. 1 Broadly speaking, these variations in 

 mitotic figures may be reduced to three main types : (i) Forms with 



1 See Wilson, The Cell, Chap. II. 



