CONNECTIVE TISSUE IN AMPHIBIA 465 



fine fibrillation. The process of transformation into a fibrous 

 tissue is a progressive one. The fibrillation increases, bundles 

 of fibers are formed and in time the entire ground substance, 

 which at first showed such a high degree of homogeneity, becomes 

 transformed into a fibrous tissue. It is indicated that this trans- 

 formation occurs as the results of the introduction of mechanical 

 factors in the embryo. These factors may be due to certain 

 lines of tension in the embryo corresponding to the inherent 

 polarity of the organism or, just as in the plasma clot, the move- 

 ments of the cells through the ground substance may introduce 

 mechanical factors which aid in the transformation of the ground 

 substance into a fibrous tissue. The cells, however, are to be 

 regarded primarily as assimilative and secretive agents, chiefly 

 concerned in the formation of the undifferentiated ground 

 substance. 



SUMMARY 



1. The primitive forerunner of connective tissue in amphibian 

 embryos is a gelatinous material (primitive ground substance) 

 which can be demonstrated around the notochord soon after it 

 is formed and shortly before the embryo has reached the tail 

 bud stage. A little later this material, which is to form in general 

 the ground substance of the connective tissues, surrounds the 

 medullary cord and a layer of it, following the body wall, extends 

 ventrally on either side and in time completely encircles the 

 body cavity. The formation of this matrix around the noto- 

 chord takes place before there is any syncytium of the mesen- 

 chyme cells in this region. It is evident, therefore, that this 

 primitive ground substance of connective tissue has arisen as 

 an intercellular secretion of the embryonic cells and not by a 

 cytoplasmic syncytium. 



2. The ground substance having been formed, cells begin to 

 move into it and wander through it. These are spherical at 

 first, but as they move through the ground substance, they soon 

 change into various shapes, becoming stellate, spindle-shaped, 

 etc. The study of the sections shows that, in general, individual 

 cells do not separate from the cell masses and move out into the 



