424 



BULLETIN OP THE BUREAU OP FISHERIES. 



the yolk sphere. After fertilization has taken place this layer of protoplasm becomes 

 concentrated in a typical manner to form the blastodisc (fig. 44, bd). The protoplasmic 

 movements involved in the process of concentration can not be satisfactorily observed 

 by reason of the opacity of the yolk. The fully differentiated blastodisc is relatively 

 thick and covers a relatively larger area of the surface of the yolk than is the casein the 

 more typical eggs described above. It thins out gradually toward the periphery and 



GOBIOSOMA BORCI. 



Fig. 43. — Mature unfertil- Fig. 44. — Egg with fuUy developed blastodisc 

 ized egg. X 5°- (bd). 



Fig. 45. — Egg with blastoderm of 2 cells. 



fades away almost imperceptibly into the thin layer of protoplasm which remains at the 

 surface of the yolk. 



Cleavage occurs essentially as in the eggs above described. The volume of proto- 

 plasm being relatively greater, however, the cleavage furrows become deeper and the 

 early blastomeres become more widely separated and show a more marked tendency to 

 become spherical in form. After the first act of cleavage is completed the first two 



GOBIOSOMA BOSCI. 



Fig. 46. — Egg with blastodenn of 4 cells. 



Fig. 47. — Egg with blastoderm of many cells. 



blastomeres, in surface view, appear circular in outline. The same tendency is apparent 

 also in the 4-cell stage. After the second act of cleavage is completed the first four 

 blastomeres stand out in perspective (fig. 46) as more or less isolated rounded elevations. 

 As cleavage advances the blastoderms exhibit a greater degree of irregularity than is ob- 

 served in the more typical teleostean eggs. Figure 47 illustrates an egg four hours after 

 fertilization. The blastoderm is now circular in outline but remains relatively thick. 



As cleavage advances the blastoderm becomes distinctly dome-shaped and a small 

 cleavage cavity becomes apparent. The periblast appears relatively thick, but can not 

 be satisfactorily observed in the living material by reason of the opacity of the yolk. 

 The phenomena involved in the formation of the germ ring and the early differentiation 



