388 T. H. MORGAN AND UME TSTJDA. 



tage, it seems to me, as compared with the more common 

 method of injuring the upper or black cells. 



Owing to the great thickness of the lower wall of yolk-bearing 

 cells there is no chance of breaking into the segmentation 

 cavity or archenteron. As the white cells seem to be the more 

 passive cells during development, injury to them has less 

 serious consequences for the developing embryo. 



The eggs were stuck at the time when the blastopore first 

 appeared, and a sketch made in each case to indicate the dis- 

 tance of the point of injury from the blastopore. A series of 

 these eggs were prepared in which the injuries were at varying 

 distances from the dorsal lip of the blastopore that had just 

 appeared. A series of figures were drawn from time to time 

 to show the relations between blastopore and point of injury. 

 Moreover duplicates of each lot were followed. Inasmuch as 

 all the experiments gave similar results, I think any doubt as 

 to abnormality caused by the operation is removed. 



If the egg (embryo) be turned with its white area uppermost 

 at the time when the blastopore first forms, so that the blasto- 

 pore just appears above the horizon, it will be found that the 

 white area does not cover quite a hemisphere of the egg. A 

 border of dark pigment appears around the periphery of the 

 white, as shown in outline by PL 25, fig. 9. The primary pole 

 of this white area (hemisphere) lies not quite in the centre of 

 the white, but nearer to the side where the blastopore has 

 appeared, as shown in figs. 1, 3, and 4. The ''centre^' of the 

 white area does not, therefore, correspond with the " lower 

 pole.^' 



The experiments here recorded were made on Rana, sp. ? 



The X shows the point where the egg was stuck. 



Experiment I (figs. 10 — 12). — Egg in which the blasto- 

 pore had just appeared. Pricked at 4 p.m. on opposite side of 

 white area, i. e. nearly a hemisphere away from blastopore. 

 At 8 p.m. the blastopore has become more arched, and the dis- 

 tance between the point injured and the dorsal lip of the 

 blastopore is much less than at first. The dotted line running 

 out from the ends of the blastopore marks the rather sharp 



