EMBRYOLOGY AND LARVAL DEVELOPMENT OP TELEOSTEAN FISHES. 427 



Blastodisc. — As soon as fertilization has taken place the protoplasm becomes con- 

 centrated to form the blastodisc (fig. 55, bd). The process of concentration can hardly 

 be described as a "streaming" of the protoplasm toward one pole of the yolk sphere, 

 but rather as a thinning- of the layer of protoplasm on one side of the yolk and a cor- 

 responding thickening on the opposite side which pushes the yolk out into an extremely 

 eccentric position. The fully differentiated blastodisc covers approximately half 

 the area of the surface of the yolk. The other half remains invested by a very thin 

 layer of protoplasm, which is continuous with the blastodisc. 



Segmentation. — Early cleavage takes place essentially as in the more typical tele- 

 ostean eggs, but advances more rapidly. The first act of cleavage occurs approxi- 

 mately 30 minutes after fertilization and the successive acts follow each other in rapid 

 succession. Because of the disparity of yolk the figures presented by the early cleavage 

 stages differ widely from those presented by the corresponding stages of the more 

 typical eggs. The first cleavage plane cuts deeply into the blastodisc and divides it 

 into two blastomeres of approximately equal volume (fig. 56). They are usually 

 quite symmetrical but may show considerable variation. The second cleavage plane 

 cuts the first at right angles. Figure 57 illustrates a typical blastodenn of four cells 

 viewed through the transparent yolk. The blastomeres are quite symmetrically 



Fig. 5S. — Egg with 

 blastoderm of 8 

 cells. 



CTENOGOBIUS ST1GMAIICD3. 



Fig. 59. — Egg witli 

 blastodenn of 16 

 cells. 



Fig. 60. — Egg with 

 blastoderm of 

 many cells. 



arranged and are seen to extend beyond the periphery of the yolk. They remain 

 continuous with the thin layer of protoplasm by which the latter is invested. 



The early blastoderm usually spreads widely over the surface of the yolk, the 

 blastomeres being arranged in a single series. Figure 58 illustrates an egg with a blas- 

 toderm of 8 cells which deviates somewhat from this condition, the blastomeres appear- 

 ing heaped up at one side of the yolk. Figure 59 illustrates an egg with a very typical 

 blastoderm of 16 cells. The cells remain in a single series and the yolk sinks deeply into 

 the concavity of its inner surface. 



As cleavage advances beyond the 1 6-cell stage the cells no longer remain in a single 

 series but become heaped up on one side of the yolk. Eggs in advanced stages of cleav- 

 age are illustrated in figures 60 and 61. 



During the later stages of cleavage the granular protoplasm on the surface of the 

 yolk at the margin of the blastoderm becomes somewhat more conspicuous (fig. 61). 

 This slightly thickened zone in the layer of protoplasm investing the yolk doubtless 

 represents the periblast. It does not become differentiated into a well-marked ridge, 

 however, and nuclei were not apparent in it. 



As cleavage advances further the blastoderm becomes more distinctly dome-shaped, 

 its central area becomes appreciably thinner, and the periphery advances round the 



