219 



enteric lining. I shall now describe some of the observations, which 

 give the basis for these generalizations. 



In Figs. 3, 4, 5 (a, b, c) are represented successive appearances 

 of the same egg. The egg was compressed a few minutes before 

 7,30 P.M. The dorsal lip had just begun to make its appearance 

 at 7,30 P.M. (Fig. 3). It is shown again at 8 P.M. in Fig. 4, and 

 at 9 P.M. in Fig. 5 a. One of the ectoderm cells next the lip is 

 shown at e. c. in Fig. 5 a. At this time it is four sided, and 

 measures 44 /< in its long axis. The cell was kept under continuous 

 observation. Its distal (away from lip) end gradually approached 

 the blastopore lip, and by 10 P.M. (Fig. 5 b), the cell had also 

 changed in shape, being now triangular with a base 33 ^i and an 

 altitude of 22 ^/. The apex of the triangle continued to approach 

 the lip, and by 10,10 P.M. the cell had completely disappeared, 

 having as I infer been pushed round the blastopore edge as a con- 

 sequence of the growth in area (doubtless due to cell proliferation) 

 of the ectoderm layer, or at any rate of that port of the layer ad- 

 joining the blastopore lip. — Another ectoderm cell in a similar 

 position is shown at e. c in Fig. 5 c (10,40 P.M.). The nucleus 

 is at the distal end of the cell, and is 44 {.i from the blastopore lip. 

 The nucleus and distal end of the cell steadily approach the lip. 

 The cell appears about square at 11 P.M., and by 11,30 P.M. the 

 nucleus had reached the lip. — In Fig. 5 c, two yolk cells, a and &, 

 are figured. The blastopore lip at 10,40 P.M. is 22 ^i from the 

 boundary line between these two cells. The lip gradually creeps over 

 the yolk, shutting out cell h from sight (in 10 min.), and then cell a. 



— Summary : cells were observed to disappear round the dorsal lip ; 

 dorsal lip covered up yolk cells. In the same experiment, ectoderm 

 cells were observed to disappear round the ventral lip, and yolk cells 

 to be covered up by the ventral lip. 



In Fig. 10 is shown an egg in which the ventral lip is forming 



— an incomplete line of pigmented depressions marks it out. The 

 figure was drawn at 10 P.M. The egg was compressed at 8,45 P.M. 

 The dorsal lip at this time had made its appearance, extending be- 

 tween points 1 and 2 in Fig. 10. The dorsal lip continued to occupy 

 this position, close to the egg periphery, until 10,30 P.M., when 

 (probably owing to the slides becoming looser) a slow rotation of 

 the whole egg began, bringing the blastopore into a central position 

 by 12,45 A.M. By 10 A.M. the next morning, the blastopore was 

 small (Ve diameter of whole egg) and excentric (egg diameter 

 being 95 micrometer divisions, the dorsal lip was 54 divisions 



