THE TISSUES. 25 



are placed either rounded and completely separated or filiform and 

 branched cells, which are united to one another by their processes. 

 Chords or tracts of cells also occur. In this way a fine network is 

 formed, which traverses the gelatinous portion of the structure, the 

 trabecular of which may become firmer by further differentiation, 

 and may become broken up into fibrillar. A similar fibrillation may 

 affect the intercellular substance, in which case fibrous bands, in 

 which there are no cells, can be made out. This tissue is found 

 in many of the lower animals; in the umbrella of the Medusas 

 (Fig. 8), the integument of the Heteropoda, &c. 



3) Fibrous connective tissue may be regarded as a further 

 development of gelatinous tissue. Its morphological elements are 

 elongated or branched cells, embedded in an intercellular substance 

 formed of fibrous tracts and bundles. This substance is largely 

 due to a differentiation of the walls of the cells, as is clear from 

 its development. Development also reveals the fact that part of 

 the protoplasm which sends off processes, is directly differentiated 

 into fibrils and fibrous bundles ; these are therefore distinct from 

 the earlier formed, and more or less homogeneous intercellular 

 substance. The thickness of the fibres and the direction they take 

 vary greatly. The fibres, which are generally curved and undu- 

 lating, sometimes run parallel to one another, sometimes anastomose; 

 the cells and the cell-processes are, in their earlier stages, arranged 

 in a manner corresponding to the subsequent arrangement of the 

 fibres. 



Fibrous connective tissue is distinguished as loose, or firm, 

 according to the characters of its intercellular substance ; the 

 firmer sort is also known as " tendinous tissue/' the fibrous bands 

 of which are placed parallel to one another. In addition to the 

 fibrilla?, which swell up when treated with acids and alkalies, 

 there is another form of fibre, which is seen in the intercellular 

 substance of fibrous connective tissue; this resists these agents 

 more completely, and is called "elastic tissue," on account of its 

 elasticity. It is, as may be seen from its relation to the inter- 

 cellular substance, not an independent form of tissue, but merely 

 a modification of connective tissue. 



Inasmuch as a portion of the intercellular substance arises by 

 subsequent differentiation of the protoplasm of the original cells, 

 as was remarked above, the morphological elements which are 

 present in fully-developed connective tissue represent the remains 

 only of the primitive cells. According to the quantity of pro- 

 toplasm used, and converted into fibrous structures, and so in- 

 corporated into the intercellular substance, the nucleus of the 

 connective-tissue cells is surrounded by more or by less protoplasm, 

 or the whole protoplasm disappears ; the presence of isolated nuclei 

 in the fibrous bands of connective tissue is an indication of this. 

 Where the protoplasm still remains around its nucleus — where, 

 that is to say, a cell, according to the conception given above, is 

 present, this cell may undergo fresh changes, which are of so many 



