132 JEREMIAH S. FERGUSON 



proximal, the round cells in the distal zone of the fin. Hence, 

 one follows the sequence of development in passing from the dis- 

 tal toward the proximal portion of such a. fin. Older embryos 

 show the same zones of transition but in them the formation of 

 fibers in the proximal region is more advanced. 



In mammalian tissue one finds three stages in the histogenesis 

 of connective tissue, a primitive cellular stage, a syncytial stage, 

 and a fibrous stage. The first is characterized by the predomi- 

 nance of round cells, the second by stellate, the third by spindle 

 and lamellar cells. The same succession of cell types is present 

 in the fins of embryo fish and there is a corresponding succession of 

 histogenic stages. Fibers do not appear prior to the appearance 

 of cellular processes. Fine fibers appear coincidently with stel- 

 late cells, coarse fibers and fiber bundles develop later. 



In the distal portion of the fin fine fibers first appear in the round 

 cell area coincidently with the transition from round to early 

 stellate forms. At exactly this period I have observed the first 

 indication of motion, the throwing out of pseudopods by the round 

 cells, in the connective tissue cells of the living embryo. Fig. 1 

 shows such changes in two cells on the border of the round cell 

 area near the posterior end of the ventral fin. There is at this 

 time relatively little locomotion, as is shown in the figure by 

 referring the position of the cells a and b to the relatively fixed 

 point, a prominence on the margin of an adjacent chromatophore 

 (chj. 



The first appearance of fibers in the distal portions of the fins 

 has been very properly connected by Harrison, and by Goodrich 

 with the origin of the dermal fin rays from the 'scleroblast' cells 

 which closely resemble the connective tissue cells and like them 

 are of mesodermal origin. In the region of the actinotrichia 

 in the distal portion of the fin, it is difficult to distinguish between 

 the early forms of these coarse fibers and the true connective tis- 

 sue fibers, but the actinotrichia are confined to the region of the 

 last one or two joints of the jointed fin rays, and there they pro- 

 ject, as Goodrich has shown, from between the two opposed 

 dermal plates which form the distal section of the jointed fin ray. 

 If therefore one studies a region proximal to the last section and 



