540 



EMBRYOLOGY. 



In a homogeneous, soft, quite transparent matrix, whicli chemically 

 considered contains mucous substance or mucin, and therefore does 

 not swell in warm water or acetic acid, there lie at short and regular 

 intervals from one another numerous cells, which send out in all 

 directions abundantly branched protoplasmic processes and by means 

 of these are joined to each other in a network. 



In the lower Vertebrates the gelatinous tissue persists at many 

 places, even when the animals are fully grown ; in Man and other 

 Mammals it early disappears, being converted into two higher forms 

 of connective substance, either into fibrillar connective tissue or into 

 cartilaginous tissue. The first-named arises in the gelatinous matrix 

 by the differentiation of connective-tissue fibres on the part of the 

 cells, which are sometimes close together, sometimes widely scattered. 

 These fibres consist of collagen and upon boiling produce glue. 

 At first sparsely represented, these glue-producing fibres continually 

 increase in volume in older animals. Thus transitional forms, which 

 are designated as foetal or immature connective tissue, lead from 

 gelatinous tissue to mature connective tissue, which consists almost 

 exclusively of fibres and the cells which have produced them. This 

 is capable of a great variety of uses in the organism, according as its 

 fibres cross one another in various directions without order, or are 

 arranged parallel to one another and grouped into special cords and 

 strands. Thus, in connection with other parts derived from the germ- 

 layers, it gives rise to a great variety of organs. In some places 

 it forms the foundation for epithelial layers of great superficial 

 extent ; together with them it produces the integument, composed 

 of epidermis, corium, and subcutaneous connective tissue, and the 

 various mucous and serous membranes ; in others it unites with 

 masses of transversely striped muscle, and arranges itself under 

 the influence of their traction into parallel bundles of tense fibres, 

 furnishing tendons and aponeuroses. Again at other places it 

 takes the form of firm sheets of connective tissue, which sfrve for 

 the separation or enveloping of masses of muscle, the intermuscular 

 ligaments and the fasciae of muscles. 



The second metamorphic product of the primary mesenchyme, 

 cartilage, is developed in the following manner : At certain places 

 the embryonic gelatinous tissue acquires as a result of proliferation a 

 greater number of cells, and the cells secrete between them a carti- 

 laginous matrix, chondrin. The parts which have resulted from 

 the process of chondrification exceed in rigidity to a considerable 

 ■extent the remaining kinds of sustentative substance, the gelatinous 



