THE FROG 37 



a way similar to that already described, and so produces the 

 perichondral bone outside the cartilage. Calcification or the 

 deposition of lime salts has already occurred to a certain 

 extent in the cartilage. While the perichondral bone is forming, 

 the formative layer sends processes inwards, which first destroy the 

 cartilage by means of special cells, the osteoclasts, and then lay 

 down strands of matrix, around which bony spicules are formed. 

 The bone thus formed in the actual position of the cartilage 

 is termed endochondral. Ossification continues until peri- and 

 endochondral bone have joined up and the whole of the original 

 cartilage has been replaced. We are now in a position to under- 

 stand a little more clearly what is meant by the term " epiphysis." 

 In a large number of bones, particularly the long bones, the process 

 of ossification starts from three places, the middle and each end, so 

 that the bone really consists of three pieces, which grow together by 

 an interlocking of the bony spicules. This interlocking is not 

 always complete, so that in a dried skeleton the epiphyses may 

 fall off. 



Connective Tissue. 



Another tissue may conveniently be considered here, 

 although it cannot strictly be considered as forming part of the 

 skeleton. It helps to bind the various soft parts to one another 

 and to the skeleton, and is therefore called connective tissue. In 

 Rana, although it is present in small quantities between the fibres 

 of the muscles, we do not find the simplest kind of connective tissue 

 nearly so plentifully as in the rabbit, where it may be obtained 

 readily immediately under the skin. Under the microscope the 

 sub-cutaneous or areolar tissue appears as a network of interlacing 

 fibres and a number of more or less isolated cells contained in a 

 colourless matrix. The cells, termed connective tissue corpuscles, 

 are of at least three different kinds. The lamellar cells are flattened, 

 have a large nucleus, and give off a number of processes, which may 

 unite with the similar processes from neighbouring cells. The 

 granular cells are more regular in shape with a well-marked nucleus, 

 and owe their name to their granular contents. The granules are 

 coarse and stain deeply with eosin and certain other acid dyes, so 

 that they are often termed eosinophilous. The vacuolated cells are 

 frothy in appearance, owing to the number of minute vacuoles 

 present in them. 



In addition to these cells, more or less constant constituents, 

 others may be present. A certain number of leucocytes are almost 

 always to be found in the frog pigment cells also. 



Two sorts of fibres will be easily distinguished crossing and 



