OF CARTILAGE. 23 



cells is subsequently absorbed. After the cartilages of the 

 branchial rays of fishes have been exposed to the action of water, 

 it is only in the young cells that the nuclei are visible ; they 

 are much more rarely seen in those cells of which the walls are 

 already very much thickened. In many cells of the branchial 

 cartilages of the tadpole, a small nucleus with a ragged outline 

 may be observed, which is probably the cytoblast of the cell 

 in the act of undergoing absorption. These cytoblasts (nuclei) 

 of the true cartilage-cells always lie in the cell-cavity, even 

 when its wall is thickened, and it is impossible to distinguish 

 whether they lie free or are still connected with the cell-wall. 

 A twofold explanation is here possible : either the cytoblast 

 separates from the wall after the formation of the cell-membrane 

 is perfected, and falls free into the cavity (as occurs in plants), 

 and at such period a secondary deposition of substance upon 

 the cell- wall first commences ; or the thickening of the wall is 

 due to an actual increase of the original cell-membrane, and in 

 that manner the nucleus is pushed inwards, and may remain 

 in connexion with the wall. If a secondary deposition of 

 substance took place before the nucleus was disengaged from 

 the cell-membrane, that body must be enclosed in the wall, 

 and would not lie in the cell-cavity. As both these expla- 

 nations are possible, it will be seen that no conclusion can be 

 drawn from the position of the nucleus, as to whether the 

 thickening of the cell-wall be a secondary deposition, or an 

 actual growth of the cell-membrane. Sometimes a carti- 

 lage-cell presents more than one nucleus; when in such a 

 case the original nucleus of the cell is absorbed, all those 

 observed are probably the germs of new cells, which have not 

 as yet commenced their development. The same fact is fre- 

 quently observed in plants. The nuclei in the branchial 

 cartilages of the tadpole have for the most part the same size j 

 some, however, which are probably not as yet perfectly formed, 

 are smaller than others. It also often occurs that a nucleus 

 is seen expanded to three or four times the usual size ; such 

 instances might be mistaken for young cells without nuclei, 

 but they may be readily recognized by their general aspect. 

 They are more transparent and delicate, and exhibit one or 

 two nucleoli, which are easily detected j when two are present 

 they are widely separated from one another. According to 



