40 THE PELAGIC STAGES OF YOUNG FISHES. 



ularities in the early stages of development, Brook concluded that, " directly 

 or indirectly, an abnormal development of the oil-globules has its influence 

 on the development of the embryo." It is evidently assumed that the normal 

 number of globules is one for all stages of the egg ; but the author has not 

 given satisfactory grounds for this opinion, and what he has given appears to 

 us to furnish some foundation for thinking that the normal number is vari- 

 able for the earlier stages, and one for the later stages. Our observations on 

 this and other species suggest this view, but do not give the data necessary 

 to decide the question. 



The earliest indications of pigment-cells are seen while the last trace 

 of the blastopore is disappearing (Fig. 1). At this time a few small, more 

 or less angular, colorless cells appear in the posterior region of the embryo, 

 and these are at first free from pigment. They increase rapidly in number, 

 becoming slightly dusky from the presence of minute pigment-granules as 

 the closing of the blastopore becomes complete. The development of the 

 chromatophores progresses from behind forward, reaching the posterior 

 boundary of the head in the course of six hours, as shown in Fig. 2. At 

 this time the anterior chromatophores are the youngest, and still without 

 pigment, while those at the posterior end are larger, amoeboid in form, and 

 already blackish with pigment. Between these two points all intermediate 

 stages of growth are seen. Anteriorly, the pigment is confined to the 

 dorsal surface of the embryo; posteriorly, it extends beyond the embryo 

 to the yolk-sac and the oil-globule. The ventral side of the embryo and 

 yolk-sac is without pigment-cells of any description. 



It is about this time (forty-eight hours after being laid) that the pectorals 

 make their first appearance, as lateral muscular thickenings, at a little dis- 

 tance behind the head. These thickenings correspond in length to two 

 segments, and each is more or less plainly marked off into an anterior and 

 posterior half, the line of division corresponding to the dividing line of the 

 segments from which they originated. 



.Fig. 3 represents the same egg twenty-four hours later. The black 

 pigment now appears rather coarsely branched, thickly scattered along the 

 whole dorsal surface of the embryo. About a dozen chromatophores are 

 now seen on the dorsal side of the yolk-sac. along the sides of the embryo. 

 None are found on the ventral half, except around the oil-globule. Most 

 of the chromatophores arc wholly or in part jet-black; along the back some 

 are lead-gray, giving the embryo a dusky or inky hue. 



