In conclusion, we must notice the metamorphoses of C. The ovum itself is, as we have already mentioned, merely a nucleated cell; after impregnation, a number of sec ondary C. are formed within it, by a process of cleavage or segmentation. See articles GENERATION Lnd Ovum. Some of the C. which occur in the ovum in its early stages soon coalesce with others to form the higher elementary parts, which we shall shortly enumerate; others, without entering into combinations, more or less change their previ ous nature, as the horny plates of the epidermis and nails; while others, again, undergo no change of form throughout the period of their existence.
The permanent C. are arranged by Kolliker (Manual of Human Histology, translated by Busk and Huxley, 1853, vol. i. p. 47) under the following heads: 1. True Cells, which have in no essential respect altered their cellular character. These occur in the epidermis and the epithelium; in the blood, chyle, and lymph; in the glandular secretions, in the fatty tissue, in the gray nervous substance, in the glands (liver, spleen, etc.), and the cartilages. Their varieties of form and contents have been already noticed. Regarding their modes of occurrence, some are either isolated in fluids or in solid tissues; others are united by apposition, without any intervening structure, into a cellular parenchyma; while others, agAin, are conjoined by an intercellular sub stance of some kind.
2. Metamorphosed Cells. To these belong—the horny scales: flattened, polygonal, or fusiform; their membrane being fused into one mass with their contents. They occur
iu the epidermis, the laminated pavement epithelium, and the hair and nails. The con tractile fiber a: fusiform, slightly flattened, considerably elongated C., whose membrane, with its soft, solid contents, is changed into a contractile substance. They occur in the smooth or involuntary muscles. The tubules of the crystalline lens of the eye: very elon gated C., with viscid, albuminous contents. The prisms of the enamel of the teeth: greatly elongated, prismatic, and strongly calcified cells. The bone a: thickened C. (with canaliculi, or minute branching canals) which have coalesced with the matrix of the bones. 77e transversely striated muscular C.: large polygonal C. whose contents have become metamorphosed into a transversely striated or striped tissue, such as is found in voluntary muscular fiber. From these C. are formed all the different fibers, net-works, membranes, tubes, etc.; in short, all the higher elementary parts of which the animal body is composed.
For further information on C. and cell-development, the reader is referred, in addi tion to the works quoted in this article, to Leydig, Lehrbuch der Histologie des ,Venschen and der Thiere, 1857; and to Frey, Histologie and Histocheme des Menschen, 1859 (trans lated byBarker, 1870. He will find full details on morbid cell-development (the growth of C. in tubercle, cancer, and other morbid deposits) in Vogel's Pathological Anatomy of the Human Body, translated by Day, 1847; and iu Wedl's Pathological Histology, trans lated (for the Sydenham society) by Busk, 1855.