42 THE PELAGIC STAGES OF YOUNG FISHES. 



Total length 2.35 mm. 



Length of yolk-sac .55 " 



Width of yolk-sac 30 " 



Width of same at hind end .175 " 



Width of head 35 " 



The time of hatching varies, according to temperature, from three to six 

 days. The development of the pigment appears to be more backward in 

 some specimens than in those above described. In one case the newly 

 hatched fish has the eyes pigmented as in the stage of Fig. 5, and the body 

 pigment in a stage of concentration scarcely advanced to this point. In this 

 specimen the two characteristic caudal spots were present, but the patch 

 above the vent was not yet indicated. 



In the egg of this species there appears to be no Knpffer's vesicle, at 

 least none was seen in any of the stages we have described. In this respect 

 the egg differs from all the others described in this memoir. On this point, 

 Brook remarks : " Kupffer's vesicle, which in Trachinus appears before any 

 protovertebrse are formed, and long before the closure of the blastopore, does 

 not make its appearance in Motella until at or after the closure of the blasto- 

 pore, and at a time when there are at least six or eight protovertebrae. . . . 

 The vesicle consists of a solid mass of rounded cells, which increases in size with 

 the formation of the intestine, and gradually disappears again as the latter 

 is pushed backwards towards the tail." If the cluster of bodies or "rounded 

 cells" are veritable cells, it is evident that they arc not to be confounded 

 with a vesicle that never shows any trace of cell-structure. Judging from 

 their time of appearance, and from their general aspect as described and 

 figured by Brook, it seems probable that these bodies represent a cluster of 

 vesicles, analogous perhaps to Kupffer's vesicle, yet not representing it. In 

 our Preliminary Report* we have called attention to similar appearances 

 that are of common occurrence in nearly all pelagic fish eggs. We have 

 described these under the name of "secondary caudal vesicles." In appear- 

 ance these vesicles differ little, if at all, from Kupffer's vesicle, and before 

 learning the history of the latter we were in doubt about the independent 

 nature of the former. 



In the absence of anything that can be properly identified with Kupffer's 

 vesicle, this species agrees with M. mustella. There is an agreement also in 

 the time of occurrence of the eggs, and in the time required for hatching. 



* Aga*siz and Whitman "On the Development of Some 1'elagic Fish Eggs," Proc. Am. Acad. Arts 

 and Sciences, XX., L884, p. 73. 



