DEVELOPMENT OF TISSUES AND OEGANS. 35 



branes, and subcutaneous tissues, which, under certain conditions, pre- 

 serve their power of movement ; as pigment-cells, which are branched 

 cells filled, with the exception of their nucleus, with granules ; as fat- 

 cells, in which the protoplasmic cell-contents are replaced by a drop of 

 oil, "which forces the flattened nucleus against the cell-membrane; and as 

 migrator}* cells, which are formed in the spaces of fibrous tissue, and 

 which possess the power of amoeboid movement. 



The form of connective tissue in which least change is produced in 

 development is the so-called mucoid or gelatinous tissue. In some 

 of the invertebrates, as in mollusks, this tissue forms the greater part of 

 the body. It consists of a soft, semi-fluid, intercellular substance, in which 

 numerous granules and broken down membraneless cells are suspended. 

 Many of these cells possess the power of amoeboid movement. 



Prom this tissue in vertebrates the fibrillar connective tissue is de- 

 veloped by the condensation of this intercellular substance into bundles 

 of fibres, held together by an albuminous cement substance, in which 

 the forms.of the cells become much changed, becoming flattened and 

 elongated by mutual pressure until the diameter of the cell is not greater 

 than that of the nucleus. 



The elastic tissues are formed out of the fibrillar connective tissues, 

 which become so modified chemically as to yield elastin and not gelatin. 

 The fibres of elastic tissue are bright yellow in color, usually anastomose, 

 and are sometimes straight, but more often coiled up in bundles. 



The different forms of development of connective tissue, especially 

 of the intercellular substance, depend upon its different chemical meta- 

 morphoses. Thus, the gelatinous tissues owe their properties to a semi- 

 fluid albuminous body, the connective tissue proper to gelatin-forming 

 bodies, cartilage or chondrin, and the elastic tissues to elastin, etc. 



Bone is characterized by the deposit of inorganic compounds in its 

 intercellular substance. This hardened tissue then incloses the partially 

 broken down cells, which form the lacunas or bone-corpuscles, in which 

 the thickened membrane and nucleus are often visible, though they dis- 

 appear in old bones and their place is then taken by serous fluids, etc. 



The bones are also rich in vessels which traverse the Haversian 



canals. 



As the deposit of salts occurs partly from these and partly from 

 the external membrane, the bones become laminated in structure, some 

 layers being parallel with the canals, others with the exterior. 



Cartilage is characterized by a sparse intercellular substance which 

 yields chondrin, and by large cells which are often the seat of endoge- 

 nous multiplication. As in bone, so in cartilage, the intercellular sub- 

 stance becomes arranged in the form of capsules around the cells, and 

 may either remain homogeneous (hyaline) or become fibrillar (fibro- 



