220 



from the egg periphery of that side, while the ventral lip was 25 

 divisions from the periphery on its side — the distances measured 

 along a dorso-ventral diameter), and the neural groove had appeared. 

 The neural groove extended from the dorsal lip to the periphery, 



and down over the entire edge of the 

 flattened egg. In this experiment as in 

 others, the apparent position of the 

 blastopore is so influenced by rotation 

 (often very slow) that it must be dis- 

 regarded as evidence bearing upon the 

 actual movement of the blastopore lip- 

 — In Fig. 11 a, a sketch of this egg is 

 given, made V2 hour later than Fig. 10, 

 at 10,30 P.M., and showing the mid- 

 ventral portion of the blastopore lip. 

 The line 1 points to a cell boundary 

 in the ectoderm, which at the time was 

 66 fi from the ventral lip of the blasto- 

 pore. The cell boundary steadily appro- 

 ached the lip, reaching it at 11,20 P.M. 

 The similar figure 11 b was drawn at 11,30 P.M. The ectodermal 

 cell boundary c, b, which at this time was 66 ft from the lip, gradu- 

 ally approached the lip, and at 12 P.M. was only 22 /< from it. 

 Summary of experiment: the disappearance of ectoderm cells round 

 the ventral lip was witnessed. 



Fig. 10. Upper surface of com- 

 pressed egg. d.i. dorsal lip. Ventral 

 lip, V. I., is forming. Actual dia- 

 meter of compressed egg 1,936 mm; 

 of blastopore in dorso-ventral direc- 

 tion 1,232 mm. 



a 



Fig. 11. 



Fig. 12. 



In Figs. 12 and 13 (a, b, c) are represented successive views 

 of the blastopore of a single egg. The egg was compressed, with 

 the white pole up, at 5,45 P.M., the dorsal lip having just appeared. 

 The ventral lip was indicated at 7 P.M., and outlined as an edge, 

 still rough, by 7,30 P.M. (diameter of compressed egg at this time 

 was 2,2 mm, diameter of blastopore 1,119 mm). It has been men- 

 tioned that the pigment spots or depressions, which first mark out 

 the blastopore edge, are not strictly confined to the zone of cleavage 



