390 T. H. MORGAN AND Um6 TSUDA. 



the boundary line, between black and white, ready to invagi- 

 nate. The injury was made nearly in the centre of the blasto- 

 poric plug, somewhat nearer to the dorsal lip. In a later 

 stage, when the yolk-plug is smaller, the defect still lies near 

 the centre of the yolk-plug (fig. 23). It seems relatively a 

 little nearer to the dorsal lip than at first. The blastopore, 

 therefore, must close in after its circular outline is formed at 

 a nearly equal rate from all points. In this case the injury 

 was so small that the overgrowth of the blastopore could not 

 have been in the least retarded. 



The experiments recorded above are taken from a series of 

 twenty-one recorded cases, and will serve as types for the rest. 

 All the results point unmistakably to the conclusion that there 

 is an overgrowth of the lower white cells by the lips of the 

 blastopore. Moreover the experiments show the extent of 

 overgrowth of the blastopore and the relative amount of 

 overgrowth of the dorsal and ventral lips respectively. 



Examining the results more in detail, we find, if we 

 assume the point of injury to be a fixed point, that the dorsal 

 lip of the blastopore moves over the white to the extent illus- 

 trated in fig. 24. This figure is made from data of fig. 10, &c., 

 keeping the point of injury in the same position. The dia- 

 meter of the circle (representing the outline of the egg) equals 

 27 mm. The distance between the blastopore and the injury 

 equals 24 mm. From 4 p.m. to 8 p.m. the dorsal blastoporic 

 lip has moved through 8 mm., and is therefore now 17 mm. 

 from the defect. At 12 p.m. the distance travelled through 

 since 8 p.m. is 7 mm. The dorsal lip is now 10 mm. from the 

 defect. So far the blastopore has passed through 15 mm. of 

 the 24. As the defect lies outside of the point of closure of 

 the blastopore by 2 mm., the blastopore now measures 8 mm. 

 Assuming that from this time onwards the blastopore grows 

 together at an equal rate towards its centre, the dorsal lip will 

 pass over about 4 mm. more of the white. In this time the 

 dorsal lip has moved through 20 mm. of the white area. The 

 ventral lip has passed through 4 mm. 



The region in front of the blastopore covered by the over- 



