18 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Since Bothus spawns in May, I was able to get specimens which were 

 certainly not more than one month old. The one shown in frontal sec- 

 tion in Figure 14 (Plate 3) was 2 mm. long. However, as P. america- 

 uns grows much more slowly than Bothus, it is not jiossible to compare 

 ages on the basis of relative lengths. In Bothus at this stage both 

 supraorbital bars are present and there is as yet no sign of reduction in 

 either of them. In the sinistral flounder (Bothus) it is, of course, the 

 riglit supraorbital bar which disappears to give passage for the eye, 

 whereas in P. amcricanus it is the left. Since in the middle of the bar 

 its jilane slants inward and downward, and since the bar in its course 

 from ear capsule to ethmoid is also slightly convex dorsally, it is evident 

 that no one section in any plane could show the whole bar. Both bars 

 extend over the eyes, as can be seen from the position of the dotted 

 lines shown in the figure (Plate 3, Fig. 14), which represent the location 

 of the eyes, as seen in a more ventral section, accurately projected upon 

 the plane of this section. 



Appearances of degeneration in P. americanus taken after June 1 

 are rare. The youngest fish must be at least six weeks old at that time, 

 and only the most nearly symmetrical of the smallest fishes sectioned 

 show any trace of the left supraorbital bar, either normal or degenerat- 

 ing. Figure 15 (Plate 3) sliows the appearance, in frontal section, of 

 the anterior degenerating end of the posterior remnant in P. americanus 

 at Stage III. a, extending forward from the region of the ear capsule. 

 The whole section of the bar has been drawn, so as to show the difterence 

 in appearances at the two ends. The cell bodies {cl. crt.) at the anterior 

 end of the bar are much shrunken and the intercellular ground sub- 

 stance has for the most part disappeared. The nuclei are much crowded, 

 have lost the characteristic form seen in most normal nuclei, and are 

 angular and dense in appearance. 



The degenerating portion of the cartilage is darker than the un- 

 changed cartilage cells next to it. The connective-tissue sheath {tu. 

 cont. lis.) around the cartilage is, however, persistent and can be 

 traced to the ethmoid. 



In this specimen there is a coagulum filling the space in which the 

 degenerated portion of the cartilage bar formerly lay. The presence of 

 this coagulum is easily accounted for on the assumption that the sheath 

 has retained the material resulting from the degeneration of the carti- 

 lage cells, and that the killing fluid has caused it to be precipitated. 

 This condition is similar to that observed by Looss ('89) in the resorp- 



