THE DEVELOPMENT OF MOTELLA MUSTELA. 301 



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 proto- 

 vertebrse. The vesicle itself is also differently constituted. In 

 TracMnus it consists of a large single amber-tinted body, which 

 is quite transparent and shows no cell-structure with a magnify- 

 ing power of 100 diameters : whereas in Motella it consists of a 

 solid mass of rounded cells which increases in size with the for- 

 mation of the intestine, and gradually disappears again as the 

 latter is pushed backwards towards the tail. In TracMnus 

 KupfEer's vesicle disappears before the tail begins to grow free 

 from the yolk, and also before the heart begins to pulsate ; while 

 in Motella the tail has grown free some distance and got a curious 

 twist in it (fig. 6) before the vesicle disappears. In Motella also 

 the tail grows as a free prolongation for some time before the 

 heart begins to pulsate. The first formation of the heart and 

 alimentary tract takes place in Trachinus at the time of the 

 closure of the blastopore, and when Kupffer's vesicle is in- 

 creasing in size. In Motella they do not arise until the blasto- 

 pore has been closed some time, and Kupffer's vesicle has passed 

 its maximum development. Early on the third day the embryo 

 presents the appearance shown in fig. 5. The part of the yolk 

 immediately under the head then contracts, and during the next 

 twenty-four hours the boundary of the existing cavity is pushed 

 back so as to enclose the space in which the heart is formed, as 

 shown in fig. 6. Early on the fourth day it was noticed that the 

 oil-globule in all eggs had an investing membrane binding it to 

 the yolk. This probably consists of hypoblast, and was left 

 behind with the advance of the blastodermic rim over the yolk, 

 as suggested by Eyder. On the fifth day the membrane in- 

 vesting the oil-globule contains from two to five pigment-spots. 

 The embryos hatch out from 5| to 6 days after fertilization 

 under the following conditions of temperature. The tempera- 

 ture of the water in the tanks varied during the six days from 

 55°'5 F. to 51°'3, but during the daytime the temperature of the 

 water in which the eggs were developing would usually rise to 

 60°, or even 62°, and then gradually sink again during the night 

 to that of the water in the tanks. A ventral view of the newly- 

 hatched fish is shown in fig. 7, and a side view in fig. 7 a ; soon 

 after hatching, however, the continuous embryonic fin expands 

 dorsally and ventrally, as shown by the dotted lines in fig. 7 a. 

 A comparison of these figures with those of the newly-hatched 



