258 W. C. GEORGE. 



I injected the anterior cell in four eggs in which the first two 

 cleavage planes were oblique to the anterior-posterior axis of 

 the egg, giving a four-cell stage with an anterior cell containing 

 a large part of the gray crescent material, two lateral cells each 

 containing a small amount of the gray crescent material, and 

 a posterior cell containing none. Observation of the surface 

 showed that though the presence of the stain did not prevent 

 further segmentation of the injected cell, it brought about a 

 moribund condition of the cells arising from it which prevented 

 their taking part in development beyond the blastula stage. 

 In three of these eggs no blastopore closure occurred though the 

 potential blastopore was marked out by a pigmented line. In 

 the fourth egg partial closure of the blastopore did occur and in 

 the process the dying material, which consisted of a piece about 

 one fourth the size of the egg, was pinched off and left free in the 

 perivitelline space. I kept this egg under observation until the 

 fourth day after injection but it never developed any structure 

 that had the slightest resemblance to a neural plate though at 

 the time of preservation it had had cilia on the surface for more 

 than twenty-four hours and the control eggs had developed into 

 curved embryos. 



Furthermore, from eggs in which I succeeded by injection in 

 bringing about the death of the two posterior ventral cells of the 

 eight-cell stage I obtained a high percentage of neurulas while, 

 on the other hand, from eggs in which I killed the two anterior 

 ventral cells, which contained most of the gray material, I 

 obtained no neurulas. 



Further evidence that the gray crescent forms the nervous 

 system was obtianed by making injections into young blastulas 

 of R. pipiens in a manner similar to that described for R. syhatica. 

 In nine young blastulas I killed all or most of the gray crescent 

 material by making transverse injections through the gray cres- 

 cent region or by two or three radially directed injections. In 

 five of these eggs development did not proceed beyond early 

 gastrulation, but in four of them partial overgrowth of the yolk 

 by the blastopore lips took place. In the process the plugs of 

 dead material around the injected points were squeezed out of 

 the egg into the perivitelline space. I kept these for five and 



