SPECIES NO. 23. 43 



Brook has not described the young stages with a view to giving the develop- 

 ment and distribution of the pigment in successive stages, and his figures 

 therefore do not enable us to compare the two species in this respect. The 

 arrangement of pigment-spots on the tail appears to be much alike in both 

 species, but the stage seen in Fig. 6 is not shown in any figure of Brook's 

 earlier than two and a half days after hatching (see Plate X. fig. 10). 



The youngest specimen (4 mm. in length) of M. argentea described by 

 Agassiz [Joe. <■//., p. 294) differs already very widely from the latest stage 

 here described, as will be seen from the following: " It was remarkable for 

 the comparatively strong coloring for so young a stage. The head, the dorsal 

 and ventral muscular lines, as well as the sides of the stomach, are of a dark, 

 dirty yellow. The pectorals are huge and transparent; but the ventrals, 

 already well developed, are of a dark maroon color. The lower part of the 

 eye is light blue, the pupil of a dark crimson. About half-way between 

 the tail and pectorals there are two large pigment-cells, one in the dorsal, 

 the other in the ventral side of the muscular axis. A small cell indicates the 

 position where the embryonic caudal fin-rays are forming." 



Species No. 23. 



Plate XVIII. Figs. 7-10. 



The material for the stages here described was obtained during the first 

 half of July (1884). Only one or two eggs were fished up at a time, and the 

 whole number thus obtained did not exceed half a dozen. With this scanty 

 material we were unable to determine the species to which the eggs belong; 

 but the stages described may be of some service for future identification. 



The egg measures .94 mm., and its oil-globule .17 nun. In the earliest 

 stage obtained (Fig. 7) the yolk-sac is entirely free from pigment; but the 

 embryo is marked with a lew round black dots, thickest along the middle 

 region, and eight conspicuous clusters of yellow pigment-cells. The black 

 dots are confined to the dorsal surface, while most of the yellow spots are 

 on or near the ventral surface. The yellow spots invariably present the 

 characteristic paired arrangement seen in Fig. 7- The first pair is placed 

 close behind the eyes, the second a short distance behind the otic capsules, 

 the third near the middle of the embryo, just above the hind edge of the 

 yolk-sac, the fourth (dorso-ventral) midway between the third and the tip. 

 In addition to these paired spots, there is a loose cluster of yellow pigment- 



