BAIRDIELLA CHRYSURA AND ANCHOVIA MITCHILLI. 



blastodisc. In surface view the early blastomeres appear distinctly outlined periph- 

 erally (fig. 3). Viewing the early blastoderm in optical section from the side, how- 

 ever, it is apparent that the blastomeres are not entirely cut off peripherally, but 

 are continuous with the thin layer of protoplasm at the surface of the yolk. This 

 condition is illustrated in figures 25 and 27, in eggs of AiKhovia milchilli. The first 

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



BAIRDIELLA CHRYSUKA. 



Fig. I. — Egg with fully developed bias- Fig. 2. — Egg with blastoderm of 2 cells, 



todisc ibd). X 55- X ss- 



also show a decided tendency to assume a spherical form, as is indicated by the deep 

 indentations between the cells at the periphery of the blastoderm and the open area at 

 the center (fig. 3). In the 4-cell stage the two axes of the blastoderm are approximately 

 equal. 



The third cleavage furrows cut the blastoderm approximately parallel with the first. 

 When the third act of cleavage is completed and the blastoderm is composed of 8 cells, 



ph- 



BAIRDIELLA CHRYSURA. 



-Egg with blastoderm of 4 cells. 

 XSS- 



Fig. 4. — Egg with blastoderm of 16 cells; 

 pb, periblast. X SS- 



one a.xis is again distinctly longer than the other. In the i6-cell stage (fig. 4) the 

 blastoderm is usually more or less nearly circular in outline. 



While blastoderms in the early cleavage stages show considerable variation, cleavage 

 in these eggs may in general be said to proceed very regularly. The majority of the 

 blastoderms observed in the 4-cell stage were almost ideally symmetrical. The same 

 may be said of many of the blastoderms of 8 cells. At this stage irregularities are not 

 uncommon, however. A marked tendency toward regularity is apparent also in blasto- 



