BAI-RDIELLA CHRYSUR.\ AND ANCHOVIA MITCHILLI. 7 



posterior, i. e., the embryonic pole of the blastoderm. At this pole a broad tongue of 

 cells, several layers in depth, may be observed before any evidence of invagination is 

 apparent around the rest of the periphery of the blastoderm. Figure 7, plate 11, illus- 

 trates an early stage in the differentiation of the germ ring. In this blastoderm invagi- 

 nation was not yet apparent. The following figure (fig. 8) illustrates a blastoderm 

 in which the broad tongue of cells is already growing forward from the embryonic pole, 

 and the entire germ ring is well differentiated. At this stage the central area of the 

 blastoderm has become materially thinner than the peripheral area. Viewed from the 

 under side the blastoderm is now distinctly concave. Between its concave surface and 

 the periblast there is a perceptible subgerminal cavity closed in on all sides by the 

 germ ring. The blastoderm gradually increases in size by centrifugal growth. The 

 germ ring, therefore, which in its earlier stages is comparatively narrow, increases in 

 width both by the invagination of the marginal cells and by the centrifugal growth of 

 the blastoderm. 



■^ 



PP 



BAIRXHELLA CHRYSURA. 



Fig. 7. — Egg with blastoderm showing early germ 

 ring {gj). X 55- 



Fig 8. — Egg with blastoderm showing fully developed 

 germ ring ((7r) and beginning of embryonic shield (e^); 

 />^, posterior pole of blastoderm. X 55. 



While the germ ring is becoming differentiated the cells forming the surface layer 

 of the blastoderm become thin and flattened. This flattening of the surface cells is 

 less apparent in the region of the germ ring, especially in the neighborhood of the 

 embryonic pole, than in the central area of the blastoderm. In the neighborhood of 

 the embryonic pole the surface cells remain relatively thick and more or less polygonal 

 in form. 



After the germ ring is completely differentiated the blastoderm increases in size 

 more rapidly than in the earlier stages and advances around the surface of the yolk 

 sphere. The broad tongue of cells which grows into the subgerminal cavity from 

 the embryonic pole of the germ ring also increases in size, and the area of the blasto- 

 derm immediately over this ingrowing tongue of cells becomes differentiated. This 

 differentiated area represents an early stage in the formation of the embryonic shield 

 (fig. 9). 



Soon after the embryonic shield has become distinctly outlined there occurs a 

 thickening along its antero-posterior axis. This relatively opaque linear area repre- 



