14 THE PELAGIC STAGES OE YOUNG FISHES. 



egg will be required. One or two facts are worthy of note in the subsequent 

 history of these yolk-segments. They have an epibolic growth, gradually 

 expanding around the unsegmented yolk as this becomes enclosed by the 

 blastoderm and periblast. During this expansion it becomes evident that 

 the segments are not only diminishing in thickness, owing to an increased 

 surface extension, but also in absolute bulk, as may be seen by comparing 

 Figs. 3 and 6. They are visible at the time of hatching (Fig. 7), and for a 

 short time afterward. This decrease in bulk, taken in connection with the 

 nucleus-like aggregations of protoplasm seen on the outer periblastic surface 

 of the segments, suggests that the substance of these segments is gradually 

 appropriated by the periblast. In the movement of these yolk-segments 

 from one pole of the egg to the opposite side, we have ocular evidence that 

 the epibolic growth of the blastoderm is accompanied by a transposition 

 among the yolk-elements, closely analogous to the invaginatory movement 

 of the yolk in holoblastic ova. It is highly probable that this sort of solid 

 invagination is the mechanical result of the peripheral expansion of the 

 blastoderm, and that it is not an exceptional feature in the development of 

 telolecithal vertebrate ova, but a general one. In the second part of this 

 work this point will receive further consideration. 



Pigment makes its appearance soon after the closing of the blastopore. 

 It is confined to the embryo and the oil-globule, and consists of round black 

 dots, from .005 mm. to .01 mm. in diameter, placed along the dorsal angles 

 of the muscle-plates. Viewed from above, these dots form two rather irreg- 

 ular lateral lines, reaching from just behind the head to near the end of 

 the body. In a profile view the dots are scattered over the whole dorsal 

 surface of the anterior half of the body, but they are more numerous along 

 the two lateral lines. The whole yolk is now enclosed by the large yolk- 

 seginents, with the exception of a broad zone along each side of the embryo. 

 It should be remembered that the area first occupied by the yolk-segments 

 corresponds, in part at least, with that now held by the embryo ; it is evi- 

 dent from this that these segments have shifted their position from the 

 dorsal to the ventral side, and in so doing have become separated along 

 a median dorsal zone. 



Shortly before hatching (Fig. 6) we find a considerable number of biwvn- 

 ish yellow chromatophores intermingled with the black in tin- first two thirds 

 of the body ; in the tail, the pigment-cells are few in number, small, and 

 all black. At the time of hatching (Fig. 7), — about thirty-six hours after the 



