6 S. F. CLARKE. 



about the pole are larger than the others. Very soon an irregular, 

 slight depression of this polar region occurs, which lasts but a 

 short time; this area becoming again even with the surface of the 

 sphere. But this movement or action has resulted in the forma- 

 tion of a very narrow, even, clearly marked groove, which sharply 

 defines, or makes a distinct boundary to, the polar portion of the 

 white area, and extending on either side along the edge of that 

 portion of tiie lighter area which stretches away from the pole, 

 gradually fades out. See Plate 1, Figure 1. Now the curved 

 groove becomes less and less widely open in front, (Plate I, Figure 

 2), until finally the two ends meet. The groove around the now cir- 

 cular area becomes gradually deeper, the entire surface outside of 

 the circular, j)olar area has become dark colored ; the polar-area 

 itself is composed of large white masses with dark outlines. Id 

 this way is formed the " vitelline plug" of Ecker. Plate I, Figure 

 3. In a side view of an egg a kw hours after the formation of the 

 vitelline plug, one sees that the latter has become raised up from 

 the surface of the egg, giving the appearance of a small white mass 

 resting on or protruding from a dark colored sphere. Plate 1, 

 Figure 4. A front or polar view of the same egg at the same 

 period is shown in Plate 1, Figure 3. The plug retains this 

 prominent position but for a few hours and then begins to sink 

 into the egg; as it does so, the adjoining parts of the egg close 

 around it until there is a very small, circular pit or depression 

 left in the centre of the area formerly occupied by the vitelline 

 plug. While the plug is thus being withdrawn into the egg, 

 there appears on nearly opposite sides of the contracting area occu- 

 pied l)y the vitelline plug, the walls of the anal part of the medul- 

 lary fold. Plate 1. Figure 5. This change has, of course, produced 

 a corresponding change in the outliue of the egg, between which 

 and the vitelline membrane there is now quite a well marked 

 s])ace, but which is greatest at the lower pole. The medullary 

 folds extend forward towards the opposite or anterior pole of the 

 egg, quite rapidly, so that by the end of the fourth day after the be- 

 ginning of the formation of the vitelline plug, a stage represented in 

 Plate 1, Figure 2, the two folds have met at the head end. Plate 1, 

 Figure 6. The cephalic portion of the medullary fold is much 

 widest and thickest and the cephalic ends of the lateral wall of the 

 medullary folds are more widely separated than the anal ends. 

 The space enclosed by the medullary folds is marked through its 



