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, symme- 
trical, 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. Miiller, 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 fcetal pigs of eight lines in length, in the form of a thick 
cord lying within the cartilaginous vertebree, its internal structure, in 
the embryos of mammalia, birds, and amphibia, being, according to 
his observations, essentially similar to the permanent analogous forma- - 
tions of the cartilaginous fishes.) Soon after this J. Miiller, from 
his own independent investigations, gave a more detailed ex- 
planation of the cells in the spinal cord of fishes (Myzxinoiden, 
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. xvii, 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. Hach 
of them, however, contains, in the mass in its interior, a dark round 
nucleus, or formation, which reminds the observer of the nucleus oc- 
curring in the cells of the epidermis, the pistil, &e., 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 
