EARLY DEVELOPMENT IN THE FROG. 349 



ment of the evidence is all the more interesting in view of the 

 fact that such gradients have not heretofore been recognized as 

 characteristic features of organic constitution." (Child, 19150, 

 p. 87.) 



2. Origin of the Dorsal Lip Region. The data on differential 

 killing and differential inhibition in the frog show, not only that 

 the apical pole region is more susceptible than other parts of the 

 egg during early development, but also that as development pro- 

 ceeds a secondary region of high susceptibility appears in the 

 sagittal plane about 100 below the apical pole. This is the 

 dorsal lip region. Morgan and Boring (1903) showed that in the 

 eight cell stage, that cell of the upper quartet lying nearest the 

 grey crescent is slightly smaller than the other three cells of the 

 quartet. This inequality persists up to a stage some time after 

 the beginning of gastrulation, cell division being somewhat more 

 rapid here than elsewhere on the egg. It is well known that 

 during early cleavage some of the apical cells migrate or are 

 pushed downward to form a so-called germ ring that lies at first 

 on the equator of the egg and later below it. Correlated with 

 the greater activity of the apical pole cells in the sagittal plane, 

 this migration or pushing is most rapid in this plane, i.e., in that 

 pigmented quadrant the median meridian of which bisects the 

 grey crescent. This displacement of material from the apical 

 pole region is probably not to be conceived as an active migration 

 but as a consequence of the more active cell division and growth 

 in the embryonic meridian. After the materials in this meridian 

 have been displaced a certain distance from the apical pole they 

 seem to get beyond the range of correlation of that region; 

 certainly .they become more active and establish the posterior 

 growing region (dorsal lip). At the time of gastrulation some 

 of the material included in the dorsal, lip cells is certainly not 

 less than 100 from where it was in the four-cell stage. What 

 this migration of the materials of the apical region toward the 

 equator of the egg amounts to, of course, is a growth in length 

 which is greatest and proceeds most rapidly in the sagittal plane. 

 This dorsal lip region, which arises secondarily as a rapidly 

 growing posterior region, appears normally at a fixed distance 

 (about 130) from the apical end of the polar axis, and it is 



