6 BULLETIN OF THE UNITED STATES FISH COMMISSION, 



tra- abdominal, and the lower bulb is invested by a thinner portion of 

 the abdominal wall and adherent to the surface ui)on which the eggs 

 were originally laid. The embryos apparently remain affixed by their 

 yelk-bags until they reach the length of somewhat more than half an 

 inch, when they present nearly the form of their parents. The same 

 broad, flat, depressed, head as seen in the adult is already well marked. 

 They are also well iiigmented by this time, four broad lateral and trans- 

 verse bands of color showing on the nape and tail. By this time also 

 the inferior bulb of the yelk-sack is becoming smaller, and it is appar- 

 ent that the whole of its contents will become intra-abdominal. Soon 

 after this the young become detached from the surface to which the 

 egg adhered originally. Judging from the slowness with which the 

 early stages are passed over, I infer that the fixed condition of the egg 

 and embryo lasts for at least three or four weeks. The egg-membrane 

 is ruptured in apparently about half that time. The period of incuba- 

 tion of this species is therefore somewhere about fifteen to twenty days, 

 but the exact duration of its development was not determined, so that 

 this period is only given as approximate. 



In this species I have witnessed the origin of the pelvic fins from a 

 pair of minute horizontal folds (Figs. 2 and 3), which grow out just 

 behind the pectoral folds. They develop somewhat later than the pec- 

 toral folds and appear just about the time that the egg-membrane is 

 ruptured over the back of the embryo or when the latter bursts the 

 bonds imposed upon it by its covering. The original iiosition of the 

 pelvic fins behind* the pectoral does not last long, however, for in three 

 or four days one begins to notice that the pelvic fin folds are beginning 

 to advance and are being apparently shoved forward below the pecto- 

 rals into their iiermanent position. This is before the embryo is quite 

 three-eighths of an inch long. Bj'^ the time the young fish is a little 

 ov^er one-half of an inch in length the translocation of the pelvic fins 

 is completed. They are then inserted in advance of the base of the 

 pectorals (Fig. 1). 



I have not made any sections of these embryos, but a dissection of 

 the adult fish shows that the spinal nerves which i^ass out to the pelvic 

 fins, arise behind those which pass to the pectorals but cross the nerves 

 going to the latter and are inserted in advance of them into the trans- 

 located pelvic fin, which we saw arose originally in a position to the 

 rear of the pectoral. The paired nerves going to the pectoral are given 

 off from the spinal cord, and i)ass out just in advance of the first, second, 

 and third vertebra); those passing to the pelvic or ventral fins pass out in 

 advance of the fourth and fifth thoracic vertebrae, and a twig seems 

 also to be sent off from the third pair. We have therefore been able 

 to trace the stages of development of the nerves which pass to the 

 paired fins up to their completed state in the adult, and thus put be- 

 yond question the data upon which the doctrine of the translocation of 

 the paired fins rests. 



