DEVELOPMENT OF SKELETAL TISSUES 665 



called brown fat tissue found in certain adipose glands. It is referred to as 

 brown fat because a brownish pigment may be present in certain mammals. 

 During brown-fat formation, mesenchymal cells become ovoid in shape and 

 develop a highly granular cytoplasm. These granules give origin to small fat 

 globules which remain distinct for a time and do not readily fuse to form the 

 large fat globule, characteristic of white fat. However, they ultimately may 

 coalesce and become indistinguishable from the ordinary lipocyte found in 

 white fat. In man, this type of fat disappears shortly after birth; in the cat, 

 it is present until maturity when it transforms into the ordinary type or white 

 fat; and in the rat, it persists throughout life (Sheldon, '24). In the wood- 

 chuck, this type of fat forms the hibernating gland (Rasmussen, '23). In 

 mice and other rodents, the presence of a small amount of brownish pigment 

 is evident in this type of fat. In the young monkey, hibernating-gland tissue 

 is found in the cervical, axillary, and thoracic areas (Sheldon, '24). 



2. Development of Cartilage 



The formation of cartilage is an interesting process. During the initial stage 

 of cartilage development, mesenchymal cells withdraw their processes, assume 

 a rounded appearance, and become closely aggregated. This condition is 

 known as the pre-cartilage stage (fig. 313A). Gradually the pre-cartilage 

 condition becomes transformed into cartilage by the appearance of the inter- 

 cellular substance, characteristic of cartilage between the cells (fig. 313B, C). 

 As in the case of the connective tissues described on page 664, two schools 

 of thought explain the appearance of this intercellular substance: 



(a) as a modification of the ectoplasm which separates from the chondro- 

 blasts and 



(b) as a secretion of these cells. 



In hyaline cartilage, the homogeneous, amorphous, ground substance is 

 predominant, together with a small number of fibrils; in fibrocartilage, a 

 large number of white, connective-tissue fibers and a smaller amount of the 

 amorphous substance is deposited; and in elastic cartilage, elastic, connective- 

 tissue fibers are formed in considerable numbers. The mesenchyme, immedi- 

 ately surrounding the mass of cartilage, forms the specialized tissue, known 

 as the perichondrium. The perichondrial layer, as the name implies, is the 

 tissue immediately surrounding the cartilage. It connects the cartilage with 

 the surrounding connective tissue and mesenchyme. The inner cells of the 

 perichondrium transform into chondroblasts and deposit cartilage; in this 

 manner the cartilage mass increases in size by addition from without. The 

 latter form of growth is known as peripheral growth. On the other hand, an 

 increase within the mass of cartilage already formed is the result of interstitial 

 growth. Interstitial growth is effected by an increase in the number of cells 

 within the cartilage and by a deposition of intercellular substance between 



