ANIMAL CELLS. 981 



cells, from which, by further multiplication and differentiation, all the 

 tissues are evolved. These two cells are, originally, elementary his- 

 tological parts of the tissues of parent animals, evolved by special de- 

 velopmental processes. 



It is supposed by Schleiden, Schwann, and their followers, that all 

 organized tissues, whether animal or vegetable, are produced directly 

 from cells ; but, though this is proved concerning the vegetable tissues, 

 there are certain animal structures which, it would seem, are not 

 formed out of such cells themselves, but are developed, for example, 

 from the intercellular matrix or substance. Hence the so-called cell 

 theory is regarded by many as exploded, so far as animal organisms are 

 concerned. But if the definition of the term cell be extended, these 

 discrepancies of view may be generally reconciled. 



In a perfect vegetable cell, Figs. 46, 47, as seen detached in the vesicle of a 

 microscopic fungus, or aggregated in the section of an onion, there is found a 

 wall or periplast always composed of cellulose or lignin, with ai contained pro- 

 toplasm or endoplast, the outer part of which is often firmer than the interior, 

 and is named the primordial utricle ; in the endoplast is a nucleus, and in that, 

 often, a nudeolus; around the nucleus is collected a soft, granular, germinal 

 matter. In becoming altered, so as to constitute a vegetable tissue, these cells 

 enlarge, change their shape, the nature of their walls or contents, and their 

 connections or modes of junction, forming flat polygonal cells, polyhedral cells, 

 elongated, fusiform, tubular, or reticular tubular tissue, plain, dotted, or spiral 

 ducts, woody fibre, spores, zoosperms, pollen, or ovules. The intercellular 

 substance is always scanty. For the formation of the tissue of a growing 

 plant, such cells must also multiply ; and this is effected by division of the cells, 

 or by the formation of buds, or offshoots, which is much the same process, and 

 resembles the fission or gemmation of non-sexual reproduction ; but they can 

 also multiply, as in the spore-cases of fungi, by internal or endogenous forma- 

 tion, which more resembles the sexual mode of reproduction. In the vegetable 

 cell, the essential punctum or point, the centre of nutritive or developing force, 

 is the nucleus or the nucleolus. It is this which appears to attract materials 

 for the formation of the germinal matter around it, for the maintenance of the 

 endoplast, and for the deposition of the periplast. 



In animal cells, -far greater variety of form and plasticity of function 

 are observed. When in their most complete condition, as in the ovum, 

 they consist, like a vegetable cell, of a cell-wall or envelope, the peri- 

 plast of fluid or semifluid contents, the endoplast of a nucleus, and 

 usually of one, two, or more nucleoli. The cell-wall is a thin, delicate, 

 homogeneous, transparent membrane ; but this is absent in many cells, 

 such as the primary embryonic cells, arid the white corpuscles of the 

 blood, lymph, and chyle. The outer part of the endoplast is said 

 sometimes to be firmer than the rest, and to constitute a special in- 

 vestment, like the primordial utricle of the vegetable cell. (Huxley.) 

 In less perfect cells, there is often merely found a nucleus enveloped 

 in a soft granular mass, as in the fusiform, unstriped muscle cell. The 

 perfect cells may be called cystoplasts ; the imperfect cells gymno- 

 plasts. The cell contents of the former, or the cell substance of the 

 latter, present, in certain cases, as in the primitive embryonic cells and 

 in all newly-formed cells, besides fat particles, a peculiar semifluid, 

 transparent, tenacious, albuminoid substance, called the protoplasm, 

 which contains very minute molecules, and oleoid, and often amyloid 



