THE NERVOUS SYSTEM OF AMPHIBIA 179 



to the rent portions of the cord, it being pulled away from the 

 outer parts of the cord along with the external limiting membrane. 

 In this case also there are on both sides of the area of adhesion 

 filamentous connectives between the skin and spinal cord, and in 

 some of them yolk spherules are found. It is of particular inter- 

 est here to note, further, that there are filamentous connectives 

 across the rent of the skin, between the epithelial cells of one 

 layer and those of the other. These connectives have the 

 same structural and staining characteristics as have the con- 

 nectives between the skin and spinal cord. If our interpretation 

 of the cause of the rent in the tissue is correct, these connectives 

 across the rent in the skin were fixed by the solution in their 

 natural relations and afterwards torn from their normal setting 

 in the epithelial cells. In other words, they may be regarded as 

 intracellular structures that have been torn from their natural 

 relations after fixation. In structure and staining reactions, then, 

 the connectives between the spinal cord and skin have the appear- 

 ance of cytoplasm. 



Such adhesions as have been described between the skin and 

 the spinal cord occur, during the period under investigation, 

 only near the end of the tail bud, and particularly in the younger 

 stages. 



Figure 24 illustrates the manner of distribution of the fibers 

 of the giant ganglion cells beneath the skin in the embryo of 

 early-swimming stage. Mesenchyme cells are abundant in the 

 vicinity of this fiber but its course is largely free from contact 

 with them. Comparison of this with figures 8, 17 and 18 will 

 give some idea of the changes that have taken place in the pe- 

 ripheral relations of these fibers in the transition from the non- 

 motile to the early-flexure stage. In the later stage the individ- 

 ual fiber has wider extension or distribution than in the earlier 

 condition, although in the latter there is connection with the skin 

 on the part of these fibers throughout the extent of the seg- 

 mented mesoderm; while in the more caudal region, as has just 

 been shown, the relation between the giant ganglion cells and the 

 skin in regions of adhesions is of such a nature as to admit of 



