EARLY DEVELOPMENT IN THE FROG. 



331 



The gastrular angle at the time of the appearance of the 

 blastopore approximates 40. 



In addition to the indications of bilaterality by the appearance 

 of the grey crescent, bilaterality is made more apparent during 

 the blastula stages by the movement of material from the pig- 

 mented region toward the equator a process that takes place 

 more rapidly in the sagittal plane than elsewhere, and results 



s. c. 



s.c. 



ii 



12 



FIG. ii. Enlarged segmentation cavity. Eggs in beginning two-cell stages 

 treated as follows with KNC: m/i,ooo 24 hrs., z/5,ooo 24 hrs., w/io,ooo 12 hrs., 

 ra/2O,ooo 12 hrs. (Experiment KNC H 5.) 



FIG. 12. Small segmentation cavity as seen under conditions of severe inhibi- 

 tion. In this case eggs were exposed 48 hrs. from a two-cell stage to m/io,ooo 

 HgCla. (Experiment IV 40.) 



in the thinning out of the walls and roof of the segmentation 

 cavity, which now comes to lie nearer that side of the egg where 

 the dorsal lip appears. The so-called germ ring, formed by the 

 downward movement of animal pole material, and which lies 



FIG. 13. Obliterated segmentation cavity. (Experiment IV 56.) 



r 



at first on the equator and later below it, extends somewhat 

 farther down on the egg in the dorsal lip region. To express the 

 situation in other words: growth in length (between the apical 



