J. R. GREGG 239 



Dorsal Lip Fonnation. The first clearly visible external sign 

 that an embryo is beginning to gastrulate is the appearance of a 

 small pigmented streak to one side of the vegetal hemisphere. If 

 median sagittal sections are made of an embryo in this stage, it is 

 easy to see that the pigmented streak consists of the outermost 

 constricted ends of cells attached to the surface coat and bulging 

 at their free ends inward and upward toward the blastocoel. 

 Vogt (1929) has some exceptionally clear figures of such cells, 

 which we shall call "flask cells." The pigmented streak thus con- 

 stituted indents in the surface of the embryo, the cells just dorsal 



FLASK-CELL 

 FORMATION 



ARCHENTERON 

 FORMATION 



EPIBOLY 



Fig. 4. Morphogenetic movements in amphibian gastrulation. 



to it forming the early dorsal lip. Figure 4 contains a highly 

 schematized representation of this process. 



Since each hybrid embryo finally, if belatedly, develops a 

 normal-looking early dorsal lip, it is not surprising that Moore 

 found no irregularities of histological structure when he examined 

 median sagittal sections of Stage H10+ hybrid embryos. Flask 

 cells were present in normal fashion, attached at their outer ends 

 to the indented and pigmented area adjacent to the forming 

 dorsal lip. It is unlikely, therefore, that incapacity to execute the 

 correct preliminaries to early dorsal lip formation is instrumental 

 in bringing hybrid gastrulae to a developmental impasse. Ex- 

 periments with explant systems reinforce this view. Holtfreter 



