98 BULLETIN OF THE BUREAU OF FISHERIES. 



sense of Sumner " can not be discussed in this paper. This entire process is doubtless 

 but a part of a larger process by which much of the material contained in the embryonic 

 shield becomes incorporated in the body of the embryo. 



By the time the embryo is well formed the blastoderm covers approximately 

 three-fourths of the surface of the yolk sphere. As development advances the blasto- 

 derm soon covers the entire yolk sphere and the blastopore is closed. 



The closure of the blastopore occurs within i8 hours after fertilization. At this 

 time the embryo extends approximately halfway round the circumference of the yolk 

 sphere and segmentation of the body has already begun. Figure 9 illustrates an egg 

 shortly after the blastopore is closed. The embryo remains highly transparent and 

 shows no evidence of pigmentation. The beginning of pigmentation is observed in 

 embryos with 15 to 20 somites. The chromatophores first appear as minute rounded 

 black dots scattered over the dorsal aspect of the embryo. As the time of hatching 

 approaches, the chromatophores become somewhat larger and show irregular pigmented 

 processes. However, the embryo remains highly transparent (fig. 10). The extra- 

 embryonic blastoderm remains free from pigment. 



Larval development. — Incubation at laboratory temperature — i. e., in water at 

 approximately 22° C— occupied 42 to 45 hours. In the tidal hatching boxes at the 

 same time incubation occupied approximately 48 hours. 

 *• The newly hatched larvae (fig. 11) are approximately 2.2 mm. in length. The head 

 is slightly deflected. The yolk sac remains relatively large. It is ovate-elliptical in 

 form and free from pigment. The vent is located at some distance from the posterior 

 margin of the yolk sac and a little more than half the length of the body from the anterior 

 end. The depth of either dorsal or ventral fin fold is less than the depth of the body 

 just posterior to the vent. The chromatophores have grown somewhat larger, but have 

 not increased materially in numbers. They remain confined more or less closely to the 

 dorsal and dorsolateral aspects of the body. The fin folds and the posterior caudal 

 region of the body remain entirely free from pigment. 



One day after hatching (fig. 12) the larvae have grown to a length of 2.8 to 3 mm. 

 The yolk sac is greatly reduced and the head is no longer deflected. The chromatophores 

 have increased materially in size and show well-developed pigmented processes, but ate 

 apparently fewer in number than in the newly hatched larvae. Individual pigment cells, 

 doubtless, have become intimately associated with each other to form larger chromato- 

 phores. The larvae now have a distinctly blackish color. 



Four days after hatching (fig. 13) the larvae have grown to a length of 3.2 to 3.5 

 mm. The yolk is completely absorbed. Larvae kept in dishes of sea water as well as 

 those hatched in the tidal hatching boxes now begin to die rapidly. The critical period 

 for this species, therefore, comes about the fourth day after hatching. At this stage 

 black chromatophores are more or less uniformly distributed over the dorsal and lateral 

 aspects of the body. However, the posterior caudal region remains free from pigment. 



Figure 14 illustrates a young fish 5 mm. in length taken in the plankton. In young 

 fishes at this stage growth is indicated more especially by the increase in the depth and 

 thickness of the body than by the increase in length. The distribution of pigment 

 remains essentially the same as in larvae four days after hatching. However, the chro- 

 matophores are larger and have increased materially in numbers. 



a Sumner, F. B.: Kupffer's vesicle and its relation to gastrulation and concrescence. New York Academy of Sciences, 

 Memoirs, vol. n, pt. n, 1900, p. 47-83. 



