OF CARTILAGE.
corresponds to the wall of the corresponding cell. This appearance of strata, however, is observed only in the thick walls between two groups of cells, and as these groups probably originate by the formation of two or four cells within a parent cell, each half of the partition-wall between two groups must (presuming such to be the mode of their formation) consist again of two layers, the one of which corresponds to the wall of the parent cell, the other to that of the secondary cell, so that each partition-wall of two groups must consist of four layers. Although it does, indeed, appear that even a greater number of layers or strata are present, yet I must at the same time remark, that these observations are by no means sufficiently conclusive for the proof of a fact so important in refer- ence to the process of nutrition, and that I am so far from recarding them as evidence of a stratified deposition of the substance, as not to hold such a thing to be even probable. The appearance is probably an optical deception. As before stated, no distinction was found between primary cell-membrane and secondary thickening in the cartilages of the branchial rays of fishes, but it seemed that the cell-membrane had actually become thickened ; neither is there any such distinction to be observed in the branchial cartilages of the tadpole.
If the above described groups be assumed to have had their origin by the formation of secondary cells within a primary parent one, in that case, secondary cells which completely fill the parent one have not been developed in all the primary cells, for isolated cells occur in the branchial cartilages of Pelobates fuscus, which are somewhat larger than the secondary ones, but smaller than the other primary cells, and remarkable also, as will be seen immediately, from their contents.
The cells of the branchial cartilages of the larva of Pelobates fuscus just mentioned, contain within them one or more nuclei. (Pl. I, fig. 8, d.) These nuclei, which may be easily isolated, are either slightly oval, or perfectly globular, more or less granulous and yellowish, and apparently hollow. They contain one or two very distinct, round, dark nucleoli, which lie in their interior either close upon the wall, or very near to it. The nuclei (a portion of them at least) appear to lie free in the cell-cavity, for they may readily be isolated. The above mentioned primary cells of the larva of Pelobates fuscus in
