EMBRYOLOGY AND LARVAL DEVELOPMENT OF TELEOSTEAN FISHES. 97 



blast. A thin sheet of protoplasm, the central periblast, which is also invaded by 

 nuclei now advances centripetally from the periblast beneath the blastoderm. 



During early cleavage the blastoderm is essentially a lenticular mass of cells. As 

 segmentation advances, it becomes distinctly dome-shaped, leaving a cavity beneath 

 its central area. This cavity, which is the cleavage cavity, now lies between the 

 blastoderm and the central periblast. 



Formation of the germ ring. — The germ ring, when fully differentiated, appears as a 

 thickened peripheral zone of the blastoderm (fig. 6, gr). This zone becomes roughly 

 outlined before the marginal cells of the blastoderm are completely cut off from the 

 periblast. The thickening is at first more apparent than real, being due primarily to 

 the thinning of the central area of the blastoderm, by reason of which its under surface 

 becomes concave. After the blastoderm is completely cut off from the periblast, cells 

 at the periphery grow inward (invaginate), thus adding somewhat to the thickness of 

 the germ ring. Before invagination begins the cells forming the surface layer of the 

 blastoderm become distinctly flattened. This layer plays no part in invagination. The 

 cells which grow inward from the periphery are derived from the deeper layers. The 

 full extent of the ingrowth of cells from the periphery of the blastoderm can not be 

 determined in living material. For a detailed discussion of the r61e of invagination in 

 the formation of the germ ring and the embryonic shield based on a careful study of 

 histological sections the reader is referred to Wilson's paper on the embryology of the 

 sea bass." As the blastoderm gradually grows larger the germ ring, which in its earlier 

 stages involves but a narrow zone, increases somewhat in width by the centrifugal 

 growth of the blastoderm as well as by the invagination of the marginal cells. 



Formation of the embryonic shield and differentiation of the embryo. — Before the germ 

 ring is fully differentiated it becomes apparent that invagination advances more rapidly 

 at one pole than round the rest of the periphery of the blastoderm. This is the posterior 

 or embryonic pole (fig. 6, pp). At this pole a broad tongue of cells is pushed forward 

 into the cleavage cavity. Viewing the blastoderm from above, there soon appears at 

 the posterior pole a roughly triangular area which is obviously thicker than the adjacent 

 areas. This triangular area marks an early stage in the differentiation of the embryonic 

 shield (fig. 6, Es). 



The blastoderm now increases in size more rapidly than in the earlier stages, and 

 the germ ring gradually advances around the yolk sphere. As the blastoderm spreads 

 over an increasingly greater area of the surface of the yolk, the embryonic shield grows 

 larger and becomes more definitely outlined. Soon there occurs a linear thickening 

 along its anteroposterior axis that marks the axis of the future embryo (fig. 7, Ea). 

 The embryonic shield is now differentiated into an embryonic and an extra-embryonic 

 area. The further differentiation of the embryo begins in the anterior or head region 

 and gradually advances posteriorly. Before the embryonic axis is well differentiated, 

 the blastoderm covers more than half the surface of the yolk sphere, and the circum- 

 ference of the germ ring is actually decreasing. As development advances much of 

 the material contained in the germ ring becomes incorporated in the embryo. The 

 part played in this process by concrescence in the sense of His * and confluence in the 



a Wilson, H. v.: The embryology of the sea bass iSerranus atrarius). Bulletin United States Fish Commission, vol. DC, 

 1889, p. 209-277. 



ft His, W.; Zur Frage der Langsverwachsxmg von Wirbelthierembryonen. Verh. d. anat. Ges., 1891, p. 70-83. 



