223 



The observations in the just described experiment on the over- 

 growth of the ventral lip, though interrupted by gaps, cover a con- 

 siderable period of time; and it will be worth while to compute the 

 total observed overgrowth during this period. To the items men- 

 tioned above, must be added an overgrowth by the ventral lip of 60 jM, 

 some time between 1 P.M. and 3 P.M. — my notes lecord the 

 precise distance of the yolk cell from the ventral lip, and the fact 

 that the ventral lip approached and finally reached the cell, but do 

 not record precise times. 



Overgrowth of ventral lip, during a part of time between 1 P.M. and 3 P.M. 60 ja 



„ „ „ „ from 3 P.M. to 3,40 P.M. {40 mins.) 48 „ 

 Gap of 20 minutes 



Overgrowth from 4 P.M. to 4,30 P.M. (30 mins.) 60 „ 

 Gap of 15 minutes 



Overgrowth from 4,45 P.M. to 5,40 P.M. (55 mins.) 48 „ 



Total overgrowth observed 216 «j. 



If we select the three observations, the time dui-ation of which 

 is precisely given, and estimate from these the average rate of over- 

 growth per minute, this average rate is found to be 1,248 /<. The 

 time elapsing between 1 P.M. and 5,40 P.M. is 280 mins., and 

 accepting the estimated rate of overgrowth, the estimated total over- 

 growth by the ventral lip, during this period, is 349,44 f.i. During 

 the period from 1 P.M. to 5,40 P.M., the dorso - ventral diameter 

 of the blastopore diminishes from 1152 i^i to 552 i^i, a decrease of 

 600 ^i. One half of this is 300 /<. This is more than the actual 

 overgrowth observed (216 in), and while it is less than the estimate 

 for the total overgrowth (349,44 fi) of the ventral lip, it is near 

 enough to that estimate to justify the conclusion that during the 

 period beginning immediately (in this particular case, 15 mins.) after 

 the formation of the ventral lip, during which the blastopore de- 

 creases to one half its diameter, the ventral and dorsal lips over- 

 grow the yolk to about the same extent. If this is the case during 

 the first half of the period of blastopore closure, it very probably 

 holds for the latter half. The conclusion that dorsal and ventral 

 lips overgrow the yolk at about equal rates, is also supported by 

 the observations on the relative positions, at different times during 

 blastopore closure, of the natural mark mentioned on p. 221. 



Rotation in compressed and inverted eggs. In eggs 

 inverted (at the time of the appearance of dorsal lip) so as to bring 

 tlie white pole uppermost, the tendency to rotate back into the nor- 

 mal position acts with more or less effect on the egg, even when 

 the latter is strongly compressed between glass plates. If the dorsal 



