582 BEPORT OF COMMISSIONER OF FISH AND FISHERIES. [126] 



EXPLANATION OF PLATE III. 



Pio. 15. Cod'a egg seven days after impregnation, with the concave blastoderm fully 

 formed, and viewed from the side in optic section, and beginning to 

 spread laterally over the yelk ; a segmentation cavity sg has been de- 

 veloped, and the epiblast and hypoblast may be plainly distinguished 

 in the right-hand portion extending from A to B, the extremities of the 

 axis of the embryo lying in the embryonic disk. The floor of the seg- 

 mentation cavity is drawn too distinctly, x 50. 



PlO. 16. The same blastoderm viewed from below, showing the contour of the seg- 

 mentation cavity sg, with the embryonic disk lying below it, and embrac- 

 ing the lower half of the blastoderm, with the position of the head end 

 of the embryo fish indicated by the rounded margin of a cellular area 

 just below A, from which its axis extends to B at the margin, x 50. 



Fig. 17. Blastoderm of a cod's egg in outline of the seventh day, further advanced and 

 larger than the foregoing ; the segmentation cavity has altered its con- 

 tour but slightly, but the axis of the embryo has been somewhat elon- 

 gated from A to B. x 55. 



PlO. 18. The same blastoderm viewed in section along the axis of the embryo, show- 

 ing greater concavity below than in Fig. 15. The thickened portion 

 from A to B clearly displays the two principal embryonic layers, to the 

 right of which the segmentation cavity is seen in section, extending 

 down to the thickened rim r of the blastoderm, x 55. 



