428 



BUIylvETiN OF THE BUREAU OP FISHERIES. 



yolk. The blastoderm is now distinctly thickest at the periphery. A well-marked 

 germ ring, however, is not apparent. The yolk mass becomes constricted at the level 

 of the periphery of the blastoderm and is apparently squeezed up into the dome-shaped 

 space beneath its central area, almost entirely obliterating the cleavage cavity (fig. 62). 

 As the peripheral growth of the blastoderm advances still further the yolk is entirely 

 engulfed and the blastopore is closed. 



Differentiation of the embryo. — The closure of the blastopore occurs within six hours 

 after fertilization. Soon after this stage is reached a distinct linear thickening of the 



I^IG. 61.— Egg with 

 blastoderm in late 

 cleavage stage. 



Fig. 6:. — Egg with 

 blastoderm growing 

 round yolk, shortly 

 before closure of 

 blastopore, 5 hours 

 after fertilization. 



CTENOGOBIUS STICUATICUS. 



Fig. 63. — Egg show- 

 nig early stage in 

 differentiation of 

 embryo, 6 hours 

 after fertilization. 



Fig. 64. — Egg show- 

 ing later stage in 

 differentiation of 

 embryo. 



Fig. 65. — Egg show- 

 ing embryo well 

 differentiated. 



blastoderm, representing the axis of the future embryo, grows anteriorly from the 

 blastopore (fig. 63). Whether invagination of cells from the periphery of the blastoderm 

 plays a part in the differentiation of the linear thickening could not be determined in the 

 living material. A distinct embryonic shield was at no time apparent. As this linear 

 thickening of the blastoderm advances anteriorly, the subgerminal cavity becomes 

 apparent at its anterior extremity. As the differentiation of the embryonic axis ad- 

 vances the anterior region of the differentiated area of the blastoderm becomes dis- 



F1G.66. — Egg II hours 

 after fertilization. 



CTENOGOKIIIS STIGMATICUS. 



Fig. 67.— Hug just be- FiG. 68. — Newly hatched fish, actual length 



fore hatching. 1.2 mm. 



tinctly broader than the posterior region (fig. 64). Obviously, the differentiation of 

 the embryo begins in the anterior or head region and advances posteriorly. 



The further differentiation of the embryo advances rapidly. Within 11 hours 

 after fertilization (fig. 65) the embryo is well formed and already shows 10 to 12 somites. 

 At 12 hours after fertilization (fig. 66) the embryo makes almost a complete turn within 

 the egg membrane. The posterior region of the body Is already free from the yolk. 

 Pigment is not yet present and the embryo is highly transparent. Figure 67 illustrates 

 an egg just before hatching. The yolk mass is materially reduced. The embryo 

 remains highly transparent but is marked by small areas of delicate pigment. It now 

 makes more than a complete turn within the egg membrane. 



