8 



BULLETIN OF THE BUREAU OF FISHERIES. 



sents the axis of the future embryo. We may now distinguish an embryonic and an 

 extra-embryonic area within the embryonic shield. The differentiation of the embryonic 

 axis begins in the head region and gradually advances posteriorly until it reaches 

 the posterior pole of the blastoderm. When the embryonic area becomes distinctly 



J>P 



BAIRDIELLA CHRYSURA. 



Fig. 9. — Egg showing later stage in differentia- 

 tion of embryonic shield; qt, gerro ring; es, 

 embryonic shield. 



Fig. 10. — Egg showing embryonic shield (ci) with 

 embrj'onic area (fa) outlined; tea, extra-embry- 

 onic area; gr, germ ring; pp, posterior pole of 

 blastoderm. 



outlined it is somewhat broader in the anterior or head region than in the posterior 

 region. Observed in surface view (fig. 10) the embryonic area now has a more 

 or less regular spatulate form. While the embryonic shield is growing forward into 

 the subgerminal cavity and the embryonic axis is becoming differentiated, the germ 

 ring is continually advancing around the yolk sphere. By the time the embryonic axis 

 becomes well differentiated the blastoderm covers 

 more than three-fourths of the surface of the yolk 



(fig- II)- 



The further differentiation of the embryo ad- 

 vances very rapidly and the germ ring continues to 

 advance round the yolk until the blastoderm covers 

 the entire surface of the yolk sphere and the blas- 

 topore is completely closed. In the eggs observed 

 while the germ ring was advancing round the yolk 

 sphere the posterior pole of the blastoderm main- 

 tained approximately the same position with respect 

 to the oil globule. Inasmuch as the oil globule main- 

 tains a more or less constant position with respect to 

 the early blastoderm, it is obvious that the posterior pole of the blastoderm remains at 

 a relatively fixed point. This Wilson (1889) observed to be the case also in the eggs of 

 Serranus atrarius. In the eggs under observation the closure of the blastopore occurred 

 before i o'clock a. m. This is probably not more than six hours after fertilization. 



At the time of the closure of the blastopore the embryo extends about halfway round 

 the circumference of the yolk sphere. There is as yet no evidence of pigmentation in 

 either the egg or the growing embryo. Within one and one-half or two hours after the 

 closure of the blastopore, yellow chromatophores become sparsely distributed over the 



^■^ 



Fig. II. — Same as figure lo, lateral view. X 55. 



