PUT FORTH BY VALENTIN. 221 



semilunar, tetrahedral, or polyhedral in shape, with a mean diameter 

 of from 0*000405, to 0*000650 Paris inch. Bui bo soon aa 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 an- small grannies of 

 a round figure, with a diameter of about 0*000152 l'aris inch. The la 

 described form, was observed both by Purkinje ami myself long since 

 in the cartilages of the tadpole also, especially in the branchial arches.) 

 1 described the round cells of the globules with their interposed 

 cellular substance from the chorda dorsal is of young embryo 

 (lb. 157. Although the external appearance of the chorda dorsalis 

 clearly presents a certain resemblance to a cartilage, the microscopii 

 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 research 

 of J. Miiller, which I have noticed at page 7 in this treatise, 

 are referred to and quoted, the following also is from the sunn- 

 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 forma- 

 tions of the cartilaginous fishes.) Soon after this J. Mailer, from 

 his own independent invest ig at ions, gave a more detailed < 

 planation of the cells in the spinal cord of fishes {Myxinoiden s 

 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. xviii, 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 ami transparent 

 epithelium, the separate globules of which have the mosl 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 oc- 

 curring in the cells of the epidermis, the pistil, &C, in the vegetable 

 kingdom. In man, whose choroid plexus exhibit- a more blackish or 

 dark colour even to the naked eve, the epithelium itself has a Mimlar 

 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 bo regularly deposited, it is 



