Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/247

PUT FORTH BY VALENTIN. 221 semilunar, tetrahedral, or polyhedral in shape, with a mean diameter of from 0·000405, to 0·000650 Paris inch. But so soon as they ossify, the calcifying portion, or that which is already ossified, consists of a tissue of beautiful six-sided prisms (Balken), closely resembling vegetable cellular tissue, upon and within which are small granules of a round figure, with a diameter of about 0·000152 Paris inch. The last described form, was observed both by Purkinje and myself long since in the cartilages of the tadpole also, especially in the branchial arches.)I described the round celis of the globules with their interposed cellular substance from the chorda dorsalis of young embryos. (Ib. 157. Although the external appearance of the chorda dorsalis clearly presents a certain resemblance to a cartilage, the microscopical investigation of its structure most distinctly disproves similarity. In every instance in which it is present, it consists of an external, symmetrical, perfectly transparent envelope and globules of variable size, but always very numerous, and lying closely packed together. A gelatinous and perfectly transparent mass occupies the interspaces left between them. These globules are largest in fishes and amphibia, smaller in birds, and smallest in mammalia.” In the second passage, which Valentin cites on this point (Repertor. i, 187), the researches of J. Müller, which I have noticed at page 7 in this treatise, are referred to and quoted, the following also is from the same source :—“ which (chorda dorsalis) the reporter (Valentin) has also observed in foetal pigs of eight lines in length, in the form of a thick cord lying within the cartilaginous vertebrae, its internal structure, in the embryos of mammalia, birds, and amphibia, being, according to his observations, essentially similar to the permanent analogous formations of the cartilaginous fishes.) Soon after this J. Müller, from his own independent investigations, gave a more detailed explanation of the cells in the spinal cord of fishes (Myxinoiden,74, &c.) In the epithelia, which Purkinje and Raschkow (Meletem. c. mammal. dent. evol. 12), as well as I (Nov. act. ac. N. C. vol. xvi, p. 1. 96) — — These (the tuft-like groups of the choroid plexus) do not lie free, but they, as well as the connecting granulous membrane, are covered with a very delicate and transparent epithelium, the separate globules of which have the most regular six-sided cell-border, and are perfectly colourless and transparent. Each of them, however, contains, in the mass in its interior, a dark round nucleus, or formation, which reminds the observer of the nucleus occurring in the cells of the epidermis, the pistil, &c., in the vegetable kingdom. In man, whose choroid plexus exhibits a more blackish or dark colour even to the naked eye, the epithelium itself has a similar formation to that just described, but the centre of each cell contains in its exterior a round pigment-globule, corresponding to the central point of the situation of the nucleus in its interior. Similar pigment-globules exist in most birds, but not being so regularly deposited, it is