HISTOLOGY OF THE FROG 103 



be distinguished. The arrangement of the cells in groups is 

 the expression of the fact that they multiply by division, and 

 every group is made up of the daughter products of a single 

 cell which originally occupied a central position within the 

 group. The whole arrangement is best understood after a 

 consideration of the development of cartilage. Primordial 

 cartilage consists of an accumulation of polygonal cells 

 (derived in the first instance from undifferentiated embryonic 

 cells), each surrounded by a capsule (the product of the 

 activity of the cell-protoplasm) of a transparent gelatinous- 

 looking substance. The capsules of contiguous cells cohere 

 together, and presently become fused, thus forming a scanty 

 matrix. The cells meanwhile enlarge and undergo division, 

 and at each division the daughter cells, lying in the single 

 cavity of the parent cell, form each one a capsule for itself. 

 The secondary capsules after a time cohere to and become 

 blended with the matrix, and then each of the two daughter 

 cells again divide, and each of the four cells of the second 

 generation again forms a capsule for itself, which in its turn is 

 blended with the matrix ; and so on, as long as growth and 

 active multiplication of the cells proceeds. But after a time 

 the cells cease to multiply rapidly, and then each becomes 

 surrounded by a number of concentric capsules which are 

 successively blended with the matrix, and increase its mass. 

 Thus it is evident that the matrix is a product of cartilage 

 cells, and is probably secreted from their surfaces. 



In many forms of cartilage the cells are, as described, 

 rounded and isolated from one another in the matrix. It is 

 difficult to understand how they are nourished, for the matrix 

 is not vascular, nor can lymph channels be detected in it. In 

 other forms, however, and especially in the cartilage of sharks 

 and rays, the cells are not rounded but branched, and their 

 prolongations ramify through minute canals in the matrix, 

 and form a network of protoplasmic strands uniting the 

 cells. 



A great part of the supra-scapula of the frog consists of 

 calcified cartilage. Here we find that granules of carbonate 

 of lime have been deposited in the matrix in greater or less 

 number. The lime can be removed by acids, when the 

 structure of hyaline cartilage is made manifest, though it is 

 somewhat altered by the presence of the cavities occupied by 



