292 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



— pigment of chrornatophores, fat of the cells of adipose tissue — or 

 again in the influence exerted by the metabolic activity of the cell 

 upon the character of the matrix. This matrix is commonly 

 described as intercellular, which is quite correct, but the important 

 point is not the question whether it is inter- or intra-cellular but the 

 fact that it is in immediate relation to, and under the influence of, 

 the living protoplasm of the cell. The portion of matrix in con- 

 tiguity with one of the irregularly shaped connective-tissue cells jis 

 comparable with an intracellular vacuole the outer wall of which has 

 thinned out and disappeared. The matrix has been formed by the 

 breaking down of living substance and it seems merely a matter of 

 phraseology whether we speak of it as modified protoplasm or as dead 

 " formed " material. 



The most familiar differentiation of the matrix of connective 

 tissue consists in the development within it of thin tough fibres, 

 characterized by the physical property that they soften and dissolve, 

 yielding gelatin, under the action of boiling water, and that they 

 become further toughened by the action of tanning agents. These 

 fibres run indiscriminately in all directions or, in the more specialized 

 conditions, are definitely orientated, as in the case of tendon where 

 they are parallel and arranged in longitudinal strands, or of 

 aponeuroses where they are arranged in thin layers, those of one 

 layer perpendicular to those of the next. Other portions of the matrix 

 take the form of elastic fibres — characterized by their elasticity, by 

 their connexion together to form a network, by their being much 

 less easily affected by boiling water, and by their not yielding gelatin. 



The amount of matrix present differs greatly in different localities. 

 It may be reduced to a very small amount — to a mere demarcating 

 line — between closely fitting plate-like cells, as in the case of the 

 endothelium covering the surface of a tendon, or it may be large in 

 amount and comparatively rigid as in the case of the two great skeletal 

 tissues cartilage and bone. 



Cartilaginous or Chondral Skeleton. — The cartilage is char- 

 acterized by its cells taking on a rounded form and becoming separated 

 by an abundant semitransparent, elastic, chondrin-containing matrix. 

 The process of chondrification becomes apparent first of all in the 

 somewhat dense packing tissue (" skeletogenous layer ") * immediately 

 surrounding the notochord. This connective tissue becomes locally 

 modified to form little blocks of cartilage known as the arch-elements 

 (arcualia — Gadow, 1895), lying just outside the primary sheath and 

 arranged in four longitudinal rows, two dorsal composed of the 

 rudiments of the neural arches, two ventral — the rudiments of the 

 haemal arches. These arch-elements are apparently in the primitive 

 condition duplicated in each segment, i.e. within the limits of a 

 myotome or sclerotome there are situated two pairs of neural and 

 two pairs of haemal arch-elements. 



1 The term prechondral is applied to the young cartilage in its early stages before 

 the characteristic intercellular matrix makes its appearance. 



