EMBRYOLOGY AND LARVAL DEVELOPMENT OF TELEOSTEAN FISHES. I 23 



and in small groups posterior to the dorsal fin and just ventral to the pectoral fins. 

 Black pigmented areas occur internally along the dorsal wall of the abdominal cavity 

 and in a series at the dorsal level of the notochord. 



Figure 84 is an illustration of a specimen 33 mm. long. Scales are present, the 

 back is becoming pigmented, there is a distinct lateral stripe of dark pigment, and black 

 chromatophores are present on dorsal and caudal rays. At this stage the fish is more 

 slender than the adult. In a specimen 41 mm. long (fig. 85) the characters of the adult 

 are more apparent. The body is deeper and more heavily pigmented than the preceding, 

 and the black blotch on shoulder is distinct. 



POMOLOBUS AESTIVAUS (Miichill). BLUEBACK, GLUT HERRING. 



Spawning. — This species spawns in fresh and possibly in very slightly brackish 

 water. In some locaUties, at least, the spawning season appears to be an extended one. 

 The reason, in part at least, for this is believed to be that young fish coming to maturity 

 may ripen their eggs at a somewhat later date from the regular run of fish. Among 

 the fish taken during July in small ponds with an outlet to the ocean the majority of 

 the females were spent, a few females and many small males were ripe for stripping, and 

 a considerable number of both males and females were still unripe. Young fish 30 to 

 60 mm. in length were present in the same ponds in abundance. 



Eggs. — The eggs are demersal and somewhat adhesive, semitransparent, and 

 yellowish in color. When water-hardened they are spherical in form with a diameter 

 of approximately i mm. (fig. 87). The egg membrane is relatively thick, and its inner 

 surface appears finely corrugated. After fertilization has taken place a relatively large 

 perivitelline space becomes apparent. 



Embryology. — The volume of protoplasm in proportion to the volume of yolk is 

 considerably greater in these eggs than in the pelagic eggs already described. In the 

 unfertilized egg the protoplasm is disposed in a layer of uniform thickness investing the 

 yolk sphere. It is distinctly granular in structure; consequently, after fertilization has 

 taken place, the protoplasmic movements involved in the formation of the blastodisc 

 may be readily observed. The fully differentiated blastodisc (fig. 88 bd) is relatively 

 thick and covers a relatively large area of the surface of the yolk sphere. Near the 

 periphery it thins out abruptly and then fades away gradually into the very thin layer 

 of protoplasm that remains at the surface of the yolk. The yolk sphere now shows 

 apparent lines of cleavage that give it the appearance of being broken up into large 

 cells. This structure in the yolk is less apparent in these eggs than in the eggs of Anchovia 

 and certain other teleosts. 



Cleavage takes place very regularly and in a manner typical for teleostean eggs. The 

 volume of protoplasm being relatively large, the early blastomeres are correspondingly 

 large and show a marked tendency to assume a spherical form (figs. 89 and 90). 



As cleavage advances the radius of the blastoderm gradually increases. This 

 peripheral growth of the blastoderm becomes apparent before the periblast is well differ- 

 entiated (figs. 91 and 92). After the periblast is well formed the blastoderm grows 

 round the yolk more rapidly. When the germ ring is well differentiated the blastoderm 

 covers more than half the surface of the yolk. Figure 93 illustrates an egg in which the 

 germ ring is fully formed and the differentiation of the embryonic shield is well started. 

 The embryonic shield becomes relatively long and narrow. The embryonic axis, when 



