EMBRYOLOGY AND LARVAL DEVEOPMENT OF TELEOSTEAN FISHES. 



411 



The fully developed blastodisc (fig. 2, bd) forms a thick protoplasmic cap, circular 

 in outline, covering one pole of the yolk sphere. It is apparently of nearly uniform 

 thickness throughout the central area and thins out very abruptly near the periphery. 

 At the periphery it fades away almost imperceptibly into the very thin layer of proto- 

 plasm which remains at the surface of the yolk. 



After fertilization has taken place the egg membrane becomes slightly expanded 

 and the perivitelline space becomes more apparent. As the blastodisc becomes differen- 

 tiated the egg also becomes somewhat more transparent. 



Segmentation. — Segmentation takes place in a manner quite typical for the eggs 

 of teleosts. The first act of cleavage occurs about one and one-half hours after fertiliza- 

 tion. The second occurs less than 30 minutes after the first. The first and second 

 cleavage planes cut the blastodisc meridionally and at right angles to each other. The 

 first four blastomeres are usually approximately equal in size and quite symmetrical. 

 The cleavage furrows cut deeply into the blastodisc and the blastomeres show a decided 



CYPRINODON VARIEGATUS. 



Fig. 3. — Egg with blastodisc of 8 cells. 



Fig. 4. — Egg with blastodisc of S cells, op- 

 tical section. 



tendency to assume a spherical form. During the 2-cell stage the axis of the blasto- 

 disc at right angles to the first plane of cleavage is noticeably elongated. During the 

 4-cell stage the two axes are again approximately equal. 



As the third act of cleavage occurs one axis of the blastoderm again becomes dis- 

 tinctly longer than the other. The eight blastomeres thus formed are at first quite sym- 

 metrical, but before the fourth act of cleavage occurs much of the symmetry of the 

 blastodenn is lost and the arrangement of the cells becomes quite irregular (fig. 3). 

 Viewing the blastodenn of eight cells from the surface, the cells appear distinctly 

 outlined peripherally. Viewed in optical section from the side, however, the marginal 

 cells appear somewhat constricted at the base but are not entirely cut off peripherally 

 (fig. 4). They remain continuous with the thin layer of protoplasm which invests the 

 yolk. 



As segmentation continues beyond the 8-cell stage the arrangement of the cells 

 becomes increasingly less symmetrical. A typical blastoderm of 16 cells is illustrated 

 in figure 5. A less symmetrical blastoderm of the same stage in an egg of Lucania 

 parva is illustrated in figure 21. The blastoderm is now approaching a circular outline 

 and becomes more nearly circular as segmentation advances. 



