324 Puget Sound Marine Sta. Pub. Vol. 1, No. 28 



The periblast of the salmon egg extends completely under the blas- 

 todisk at all stages of development. The periblast of the trout described 

 by Henneguy (22) reached under the disk only slightly beyond the peri- 

 phery of the disk. 



The periblast nuclei in the salmon egg are seen in the granular mate- 

 rial at the periphery of the blastodisk in the 41-hour egg. In the 72-hour 

 egg the nuclei had undergone division and migrated under the disk about 

 half way to the center. In the 101-hour egg they were scattered through- 

 out the synctium formed by the periblast both under the disk and imme- 

 diately beyond its margin. Division of the yolk nuclei is not accompanied 

 by any division of the granular substance in which they are embedded. 



FROM GASTRULA TO EMBRYO 



As differentiation proceeds, certain changes are observed in the 

 shape of the blastodisk. Hitherto lenticular in section and slightly convex, 

 it now becomes concave in section. According to Sumner (44<, ^S), who 

 received his idea from Gotte, the change is accomplished by the migration 

 of cells, causing a thinning of the cells in the center of the blastodisk, and 

 a thickened area at the margin. A heavy band of cells is formed m this 

 manner at the periphery of the disk, enclosing a lighter central area {Fig. 

 14). The shape of this area is clearly defined even in the unprepared 

 preserved egg, since the peripheral band of cells constituting the germ 

 ring is thicker and more opaque than the thinner area on the interior. 

 Sections shows that this central area is but one cell thick in the 7-day egg. 



An increase in the diameter of the blastodisk occurs simultaneously 

 with the extension of the germ ring over the yolk. The germ ring is 

 thickened at the point where the development of the embryo begins and 

 this represents the primitive streak. Henneguy (22) describes it for the 

 trout and Wilson (49) for the sea bass. This caudal knob remains at 

 the posterior end of the embryo until the lips of the germ ring close 

 (Figs. 19-23). 



A group of undifferentiated cells, known as the secondary caudal mass, 

 is left at the rear of the embryo when the germ ring has covered the yolk. 

 According to Wilson (49), this is assimilated by the primitive streak. A 

 shallow groove marks the fusion of the lips of the germ ring in the 13-day 



egg- 



The germ ring is developed during the fifth day (Fig. 14) after fer- 

 tilization, and the following day a thickening is noticed at the point at 

 which the embryo develops later (Fig. 15). On the seventh day this 

 thickening is more pronounced (Fig. 16), and a day later the primordium 

 of the head appears (Fig. 17, 18). At this time the germ ring has ex- 

 tended one-fourth of the wav over the volk. On the ninth dav the blasto- 



