412 



BULLETIN OF THE BUREAU OP FISHERIES. 



Formation of the periblast. — As segmentation advances the blastoderm becomes 

 distinctly dome-shaped and the segmentation cavity becomes apparent beneath its 

 central area. The thin layer of protoplasm at the surface of the yolk becomes con- 

 centrated at the periphery of the blastoderm to form a somewhat flattened ridge. This 

 ridge of protoplasm gives rise to the periblast (fig. 6, pb). While the periblast is becom- 

 ing differentiated nuclei become apparent in it. As observed by Agassiz and Whitman," 

 these nuclei are doubtless derived from the marginal cells of the blastoderm. The 

 periblast is relativel}' broad and deep. The periblast nuclei are relatively numerous 

 and easily observable in the living material. 



During the earlier stages in the differentiation of the periblast the cells at the 

 periphery of the blastoderm remain continuous with it. As segmentation advances 

 farther the peripheral cells of the blastoderm become completely cut off from the peri- 

 blast. A thin sheet of protoplasm now advances centripetally beneath the segmen- 

 tation cavity. 



Formation of the germ ring and differentiation of the embryo. — The germ ring arises 

 during the later stages in the differentiation of the periblast as an apparent thickening 



CYPRINODOM VARIBGATUS. 



Fig. 5. — Egg with blastoderm of i6 cells. 



Fig. 6. — Egg with blastodenn in late cleavage 

 stage; pb, periblast. 



of the peripheral area of the blastoderm. (A late stage in the differentiation of the germ 

 ring in an egg of Liwania parva is illustrated in figure 22.) This apparent thickening 

 is due primarily, as observed by Gotte,'' to the thinning of the central area of the blasto- 

 derm and secondarily to the ingrowth (invagination) of the marginal cells. The part 

 played by invagination can not be satisfactorily studied in living material. For a 

 detailed discussion of the role of invagination in the formation of the germ ring and 

 the embryonic shield the reader is referred to Wilson's valuable paper on the embry- 

 ology of the sea bass."^ 



As the central area of the blastoderm becomes thinner its under surface becomes 

 distinctly concave. The subgerminal cavity between the blastoderm and the central 

 periblast is now closed in on all sides by the germ ring. 



Before the germ ring is completely differentiated it becomes apparent that invagi- 

 nation begins earlier and advances more rapidly at one pole than rotmd the rest of the 



° Agassiz and Whitman; On the development of some pelagic fish eggs. Proceedings. American Academy of Arts and 

 Sciences, vol. 20, 18S4. 



^ Gotte, A.: Der Keim des Forelleueies. Archiv. f. Mikr. Auat., 1873. 



c Wilson. H. v.: The embryology of the sea bass (Serranus atrarius). Bulletin, United States Fish Commission, vol. IX 

 1389, p. 309^.277. 



