664 THE SKELETAL SYSTENf 



nective tissue with its elastic fibers arise in a similar manner, with the excep- 

 tion that elastic fibers are formed instead of collagenous fibers. 



The matter of fiber formation within connective tissues has been the sub- 

 ject of much controversy. The older view of Flemming (Mall, '02, p. 329) 

 maintains that the fibers arise within the peripheral area of the cytoplasm of 

 the cell from whence they are thrown off into the intercellular space where 

 they continue to grow. However, most observers now agree that the fibrils 

 arise from an intercellular substance, i.e., from the substance lying between 

 the fibroblasts, but the manner by which this intercellular substance itself 

 arises is questionable. Some observers, such as Mall ('02) and Jordan ('39), 

 set forth the interpretation that the intercellular substance is derived from 

 a syncytial ectoplasm which becomes separated from the early mesenchymal 

 cells. Baitsell ('21) and Maximow ('29), however, consider the intercellular 

 substance to be a secretion product of the mesenchymal cells which have 

 become fibroblasts. The observations of Stearns ('40) on living material in 

 a transparent chamber of the rabbit's ear suggest that the ground substance 

 is exuded by the surface of the fibroblasts and that the fibers then develop 

 within this exudate. 



b. Formation of Adipose or Fatty Connective Tissue 



Adipose tissue is fibrous connective tissue which contains certain specialized 

 cells of mesenchymal origin, the lipoblasts. The latter have the ability to 

 produce lipoidal substances and to store these substances within the confines 

 of their own boundaries. Adipose or fatty tissue arises in fibrous connective 

 tissues in various parts of the body in proximity to blood capillaries. 



Lipogenesis or the formation of the fatty substance is an unsolved problem. 

 Two main types of fat are formed, white and brown. The process of hpo- 

 genesis in white fat, according to Schreiner ('15) who studied the process in 

 detail in the hagfish embryo, Myxine glutinosa, consists at first in liberation 

 of small buds from the nucleolus within the nucleus. These buds pass through 

 the nuclear membrane into the cytoplasm as granules or chromidia. In the 

 cytoplasm these granules appear as mitochondria. The latter increase in num- 

 ber by division. The secondary granules then separate and each gives origin 

 to a liposome which liquefies and expands into a small fat globule. Regardless 

 of the exact method by which the small fat globules arise, when once formed, 

 the small globules coalesce to form the large fat globule, typical of white fat, 

 which ultimately pushes the nucleus and cytoplasm of the lipoblast to the 

 periphery (fig. 312F). (See Bell, '09.) Lipoblasts in the mature condition are 

 fat cells or lipocytes. 



The above type of fat-cell formation occurs in the subcutaneous areas of 

 the embryo. In the human embryo it begins at about the fourth month. How- 

 ever, aside from the common type or white-fat formation, another kind of 

 fat-cell development occurs in certain restricted areas of the body in the so- 



