24 STRUCTURE AND GROWTH 



Schleiden, a similar enlargement of the nucleus also occurs in 

 plants, thus affording a remarkable accordance in what seems 

 a very unimportant circumstance. It appears to be a kind of 

 abortion ; for I have never yet seen a cell formed around such 

 a nucleus. 



The cranial cartilages of the tadpole (Plate I, fig. 9) are dis- 

 tinguished from the branchial by the smaller size of the cell- 

 cavities, and the increased strength of the firm intermediate 

 substance. The walls of the separate cells cannot now be 

 traced, they appear to have coalesced with the intercellular 

 substance, which is present in greater quantity. The cells lie 

 in groups of two or four together, and it is very probable, that 

 in this cartilage, each group is formed of cells, which have 

 been developed in a parent cell ; for some may be seen, for 

 example at c, which do not as yet quite fill the original cell. 

 Such an instance, however, is rarely so very distinct as not to 

 admit of a doubt. There is a very striking similarity between 

 the group «, fig. 9, and fig. 3, which represents four young 

 vegetable cells developed in a parent cell, and the thickened 

 walls of which have coalesced with one another and with those 

 of the parent cell, so that the four cavities onlv remain in an 

 homogeneous substance. That portion of the cell-cavities 

 which is still visible is filled with a granulous yellowish sub- 

 stance, in which lie one or more nuclei, or young cells provided 

 with a nucleus : these remains of the cell-cavities are the car- 

 tilage-corpuscles discovered by Purkinje. 



The intercellular substance is universally much more pro- 

 minent in the cartilages of mammalia than it is in those 

 hitherto described, and in them it forms the principal part of 

 the firm mass of the cartilage. There is not, however, any 

 essential difference either between the structure of the several 

 kinds of cartilage of mammalia, or between these and the car- 

 tilage of lower animals, the only distinction being that it is a 

 little more difficult to prove the existence of the special walls 

 of the cartilage-cells in the former. 



The intercellular substance in some cartilages of mammalia 

 is at first so soft, that the cells fall apart under slight pressure, 

 and float free in the fluid. If, for example, a thin lamella be 

 cut off from the cartilage at the angle of the lower jaw of a 

 foetal pig of three and a half inches in length (a period when 



