J. R. GREGG 243 



epiboly. Should this actually happen, this value would be a 

 measure not of epiboly alone but of a complex of morphogenetic 

 movements having more than one component. It may not happen 

 in hybrid embryos, for Moore says that "the tip of the archen- 

 teron maintains essentially the same position relative to the blas- 

 tocoel floor throughout. . . ." the length of time required for 

 controls to gastrulate; but it may happen in normal gastrulae. 

 But Moore's interpretation of change in the value of the angle of 

 epiboly is supported by considerable evidence. First, on his 

 interpretation, the final value of this angle for normal embryos 

 is that which would have been predicted from the knowledge 

 that, when they have finished gastrulating, the descendants of 

 their animal pole cells lie about 180° from their blastopores. Sec- 

 ond, one would expect epiboly to wrinkle the animal hemispheres 

 of hybrid embryos ( since little slack is taken up by invagination ) , 

 and this is exactly what happens. Third, there is evidence from 

 experiments with explant systems that hybrid ectoderm is capa- 

 ble of normal epibolic spreading. Holtfreter ( 1944 ) showed that 

 a piece of ventral ectoderm from an amphibian gastrula, when 

 suitably mounted on an endodermal substratum, will spread in all 

 directions over the latter. Gregg and Klein (1955) applied this 

 technique to hybrid embryos and were able to show that hybrid 

 ventral ectoderm will spread normally upon an endodermal base. 

 All this is consistent with Moore's interpretation and is just what 

 would be expected from his results. 



From our discussion, it is evident that appropriate ectodermal 

 tissues of hybrid gastrulae are capable of epiboly and, in all like- 

 lihood, exhibit it in situ. Thus we must search elsewhere for fac- 

 tors contributory to hybrid juvenile delinquency. 



Convergent Extension. As they stream toward the blastopore, 

 the presumptive notochordal cells of gastrulating amphibians 

 converge sharply toward the median sagittal plane, and after in- 

 vaginating they migrate cephalad parallel to or convergent to 

 this plane. At the close of gastrulation, therefore, the notochordal 

 cells form a narrow elongated rod of tissue lying centered in the 

 median sagittal plane (Fig. 4). 



By studying models, Holtfreter (1944) showed that normal 



