18 S. F. CLARKE. 



Figure 1 — Continved. 



ning of tbe furrow around the plug. As yet the fur- 

 row extends not more than half way around the large 

 cell area. 



Figure 2. — A view of the same side of an egsr, a few hours later ; the 

 letters as before. The furrow nearly surrounds the 

 large cell area, and the latter is changing shape. 



Figure 3. — Shows tbe same side of an ^gg, in which the fold has com- 

 pletely surrounded the area of large, light colored cells. 

 This area is now circular, and is the vitelline plug of 

 Ecker. Iietters as before. 



Figure 4. — A side view of Figure 3. The egg has contracted, leaving 

 a considerable space between it and the vitelline mem- 

 brane. This space is greatest and quite irregular in 

 the region of the plug; the latter projects from the 

 surface of the egg; v. m. the vitelline membrane. 

 Other letters as before. 



Figure 5. — A later view of the anal region; v. p. the vitelline plug 

 which has nearly disappeared within the egg; m. f. 

 the anal portion of the medullary folds stretching away 

 from the area of the vanishing plug; v. m. the vitel- 

 line membrane ; in. g. the beginning of the medullary 

 groove. 



Figure 6. — The dorsal region at a more advanced stage ; v. m. vitel- 

 line membrane; m. g. the medullary groove; m. p. the 

 medullary plate of one side; m. f. a. the anal portion 

 of the medullary fold, and vi. f. c. the cephalic por- 

 tion. 



PLATE 2. 



Figures 7 and 8 are enlarged 30 diameters. 

 The rest, 9 to 15, are enlarged 12 diameters. 



Figure 1. — A dorsal view. Letters as before. The embryo has lost 

 the circular outline of the egg and is changing shape 

 rapidly; the medullary folds have assumed an irregu- 

 lar outline, and the point at which they will first unite 

 is already indicated. 



Figure 8. — A dorsal view of the same specimen, taken two or three 

 hours later. The embryo is rapidly elongating and 

 the medullary folds have united along most of their 

 length. The sinus rhomboidalis is now one of the 



