THE CONNECTIVE TISSUES 85 



followed by bone. Again in the callus which occurs in the uniting of fractures, 

 there is first fibrous tissue, then bone, or there may be an intervening cartilagin- 

 ous stage, or repair may fail of completion, leaving permanent connective tissue 

 or cartilage. 



The correlation of the differeiu forms of connective tissue is also shown by the 

 fact that ill different species the same anatomical structure is sometimes formed 

 by different members of the group. Thus in some birds the leg tendons are 

 formed of bone, while in certain fish the skeletal system is cartilaginous. In 

 Mammals the sclera of the eye is fibrous ' acctive tissue. In Batrachians the 

 same structure is cartilage, in Birds, bone. 



Development. — All of the connective tissues have a common origin in the 

 more loosely arranged portion of the mesoderm which is known as the 

 mesenchyme. This consists at first wholly of small spheroidal or ovoid cells. 

 These cells unite to form a networkior, syncytium with closely packed nuclei. 

 As the cytoplasm increases more r ~ ' ^ly than the nuclei, the latter become more 

 widely separated. There next takes place a differentiation wliich by some has 

 been described as a differentiation of the cytoplasm into a central portion or endo- 

 plasm surrounding the nucleus, and a peripheral portion or exoplasm, by others 

 as a differentiation into cells and a primitive intercellular or ground substance. 

 Such a tissue is known as mucous or embryonal connective tissue (Fig. 30) and 

 is widely distributed throughout the embryo. It represents a stage in the 

 development of the more specialized forms of connective tissue. 



Portions of this embryonal connective tissue develop fibres. The first 

 formed fibres are known as white or fibrillated fibres or, because of their chemical 

 constitution, as collagenous fibres. Somewhat later other fibres appear which 

 are known as yellow or elastic fibres and consist of elastin. This differentiation 

 of fibres gives rise to connective-tissue proper. At other points in the mesen- 

 chyme, cartilaginous material appears. These points are numerous and widely 

 separated and each represents a chondrification center or point of development 

 of cartilage. At some points in the connective tissue or in the cartilage there are 

 depositions of lime salts — calcification. This is followed by the formation of 

 true bone — ossification. 



Regarding the development of the connective-tissue fibrils, there arc two 

 theories: (i) According to one, they are developed directly from the protop'iasm 

 of the connective-tissue cells. The cells increase in length, and each she ,vs a 

 differentiation into an endoplasmic and an ectoplasmic portion. Fine granules 

 appear in the ectoplasm, become arranged in rows and unite to form fibrils. Such 

 cells are known as fibroblasts, and their fibrils are the fore-runners of the inter- 

 cellular fibrils of connective tissue. (2) According to the other theory the fibrils 

 are developed from the matrix, minute granules first becoming arranged in rows 

 and later uniting to form fibrils. 



Regarded as opposing theories, there is in reality but little antagonism be- 

 tween them, it being probable that what has been described by some observers 

 as ectoplasm has been considered by others intercellular substance. Whichever 

 theory is accepted, the entire intercellular substance, fibres and ground substance 

 are ultimate derivatives of the cell. Recent studies, especially those of Mall, are 

 decidedly in favor of the first of the theories given above, that is of the intra- 

 cellular or ectoplasmic origin of the connective-tissue fibrils. 



