68 ANIMAL BIOLOGY. [Part I. 



A piece of tendon, by which a muscle is attached to a bone, 

 and a piece of inter-muscular tissue, the filmy glistening material 

 seen when two muscular bundles are separated, should be taken 

 from a frog's hind leg, teased apart, and mounted (a) in normal 

 saline solution, (b) in acetic acid. They may also with advan- 

 tage be stained with magenta. In those mounted in normal 

 saline, and especially in the tendon, a number of bundles of 

 ivliite fibrous connective tissue (24, vii.) will be seen. They are 

 marked with longitudinal striations, and tend to break up into 

 fibrillce. In those mounted in acetic acid the fibrous tissue has 

 been rendered almost or quite invisible. Great numbers (especi- 

 ally in the inter-muscular tissue) of fine wavy branching fibres 

 of yellow elastic tissue will be seen (24, viii.) ; and here and there, 

 with the nucleus and cell-substance more or less stained, will be 

 connective tissue corpuscles, nucleated cells, elongated in the tendon, 

 and branched in the inter-muscular tissue (24, ix.). These are 

 the living formative cells. The white fibrous and yellow elastic 

 tissue is to be regarded as inter-cellular material, analogous to the 

 cement substance in endothelium and epithelium. 



The pigment cells (24, xii. ; a. passive, b. active) of the frog's skin, 

 which, as we have already learnt (p. 7), change their form in ac- 

 cordance with the intensity and quality of the light which falls 

 on the frog's eye, are modified connective tissue cells, as are also 

 the corneal corpuscles seen (24, x.) in a section of the cornea 

 treated with gold chloride. Fat-cells (which may readily be 

 obtained from the corpus adiposum of the frog) are also strangely 

 modified connective tissue cells (24, xi.). In them the flattened 

 nucleus and a small remnant of protoplasm are thrust to one 

 side of the rounded cell, which contains a relatively huge 

 globule of oil. Treat the cells with ether ; the oil will dissolve 

 and the cells collapse. 



4. Cartilage. If the anterior or posterior cartilaginous expan- 

 sion of the sternum be removed from a recently killed frog, and 

 mounted in normal saline, rounded nucleated cells will be seen 

 occupying cavities in a transparent matrix. The cells which 

 occupy these cavities (lacunce) will be seen in various stages of 



