78 HISTOLOGY 



At the same time the fibrillated exoplasm becomes transformed into the 

 homogeneous matrix of the cartilage, which stains blue with haematoxylin. 

 Whether or not the cells are extruded may be questioned, but the rela- 

 tion of the fibrous to the homogeneous matrix, which is shown in the 

 figure, may readily be observed around the vertebrae in pig embryos. 



After the cartilage has formed, the cells occupy cavities, or lacuna, in 

 the matrix. It is probable that in the living condition the cartilage cells 

 completely fill their lacunae, but in preserved specimens they are often 

 irregularly shrunken. Usually the protoplasm of each cell is of a spongy 

 vacuolated texture, which is in part due to fat droplets and in part to 

 glycogen; in ordinary sections, both of these substances have disappeared, 

 leaving empty spaces. 



Glycogen is a carbohydrate which resembles starch and is therefore sometimes 

 called "animal starch." It is soluble in water, and soon after death it becomes 

 converted into glucose. For both of these reasons it disappears from ordinary sec- 

 tions. Fresh tissues, preserved in strong alcohol and stained with tincture of iodine, 

 exhibit glycogen as brownish-red granules which may be aggregated in masses of 

 considerable size. Glycogen is found not only in cartilage cells but also in striated 

 muscle and in the cells of the liver. In the embryo it has a wider distribution. At 

 certain stages of development, according to Gage, it occurs in the cells of the nervous 

 system and is abundant in the epidermis, the digestive tube, and the ccelomic epithe- 

 lium. Its production, like that of fat, varies with nutritive conditions, and it accumu- 

 lates in well-nourished individuals. 



The cartilage cells are said to be enclosed in capsules, which are often 

 transparent and inconspicuous linings of the lacunae. Sometimes they 

 appear as rather broad bands which are concentrically striated, indi- 

 cating that they were deposited in successive layers. The layers of newly 

 formed matrix, which bound the lacunae, usually stain very dark blue 

 with haematoxylin. The deep color is probably due to chondromucoid. 

 Peripherally the color blends with that of the older matrix, which takes 

 a pale blue stain. Like the intercellular substance of connective tissue 

 the matrix of cartilage may contain white and elastic fibers, but in its 

 commonest form it appears homogeneous and hyaline. Chemically it is 

 a mixture of collagen, chondromucoid, chondroitin sulphuric acid in com- 

 bination, and albuminoid substances (albumoid). The old term "chon- 

 drin" really means little else than the matrix of cartilage, which on super- 

 ficial examination is found to be a dense body. Within it, however, the 

 cells produce new ground substance and push themselves apart from one 

 another by interstitial growth. The cells in the interior of the cartilage 

 are often much larger than those at the periphery, and the increase in 

 the size of their lacunae is probably accomplished by the resorption of the 

 adjacent matrix. The cells divide by mitosis, and after division two of 

 them are found in a single capsule. They then move apart, and a parti- 

 tion, at first very slender, is formed between them. They may remain 



