4 Dll. G. THIN. 



substance properly so called. It is shown Imperfectly in 

 Fi-. 3. 



In the sheets of cells found free parts of them are some- 

 times seen to be formed by a double layer of cells. 



The sheets are often found of considerable size. The 

 cells represented in Fig. 2 formed about the fourth part of a 

 layer which consisted entirely of similar cells to those drawn, 

 and about a sixth part of which consisted of a double layer. 

 It may be presumed, although it is not capable of direct 

 observation, that some of the layers found free in the fluid 

 belong to a superficial layer forming the surface of the 

 cartilage. But the observation of double layers and of 

 fragments of ground-substance which with their depressions 

 are covered with cells shows that they do not all belong to 

 a superficial layer. 



The author long searched in vain for the narrow elongated 

 flat cells which he had found in potash preparations of other 

 tissues, but at last succeeded in obtaining a preparation 

 sufficiently distinct to put their existence and arrangement 

 beyond doubt. 



Fig. 5 represents rather less than the half of a mass of 

 cells, adherent to the ground-substance, isolated from the 

 head of a frog's femur in May of this year. 



The cells are in size and arrangement similar to those 

 which he had previously isolated from the mesentery of 

 the frog and from tendon, by the same process. Their 

 extreme minuteness will be estimated when it is con- 

 sidered that the figure represents as accurately as possible 

 their apparent size when examined by a No. 8 objective and 

 No. 3 ocular of Hartnack's system with the tube full out. 

 The outlines of the nucleus were sharply demarcated, but the 

 cell-substance was represented by a less clearly defined finely 

 granular substance. 



Until a more certain method of operating on cartilage by 

 the potash solution is discovered, those who would examine 

 these cells for themselves must possess in a high degree the 

 virtues of patience and j)erseverance. A strong conviction 

 of the existence of a unity of plan in the general structure of 

 the tissues, and observations on the sclerotic of the frog by 

 maceration in aqueous humour, which will afterwards be de- 

 scribed, encouraged the author to continue subjecting car- 

 tilage to the action of potash after a number of failures 

 greater than he can recount, and until he had obtained pre- 

 parations sufficiently distinct to enable him to consider the 

 existence of layers of flat cells, epithelial in their form and 



