242 GASTRULA-ARRESTED EMBRYOS 



Moore has attempted to provide us with a measure of epiboly. 

 He exploited the fact that the dorsal lip of an embryo stands in a 

 determinate geometrical relationship to the plane of its blastocoel 

 floor. This relationship can be expressed by citing the value of an 

 angle which Moore calls the angle of epiholy: it is the angle sub- 

 tended by two lines, both in the median sagittal plane and both 

 originating from the center of the blastocoel floor, one being 

 perpendicular to the plane of the blastocoel floor and the other 

 tangent to the edge of the dorsal lip (Fig. 6). The value of this 

 angle for R. pipiens embryos just commencing to gastrulate is 

 about 106°, and by the time they are in middle Stage 12 its value 

 has increased to 176° at a steady rate. For hybrid embryos the 

 initial value is 138°; with time, the values climb more slowly 

 than normal, and at a decreasing rate, but they finally reach a 

 level of about 168°. 



The problem now is to interpret those results. Moore believes 

 that the rate and amount of change in this angle are measures 

 of the rate and amount of epiboly. If this interpretation is correct, 

 then Fig. 6 yields the following information: (a) epiboly occurs 

 in hybrid embryos at a rate somewhat slower than normal but 

 to almost the same final extent, and (b) the appearance of a 

 dorsal lip in hybrid embryos is preceded by a considerable 

 amount of epiboly. In connection with (b), it should be remem- 

 bered that the dorsal lips of hybrid embryos do not appear until 

 the latter are in Stage H10+-H11~. However, it may be that 

 Moore's interpretation is correct only by chance, for although it 

 is true that the progress of epiboly will increase the value of the 

 epiboly angle, it is by no means true that an increase in the value 

 of this angle can always be construed as indicating that epiboly 

 has progressed. Any change, whether or not it is accompanied by 

 epiboly, that results in tilting the blastocoel floor away from the 

 dorsal lip will entail an increase in the value of the epiboly 

 angle. One such change that needs to be considered is the stream- 

 ing anteriorly along the archenteron floor of cells originally from 

 the vegetal pole area, for this normally occurring movement 

 might be expected to alter the plane of the blastocoel floor in 

 such a way as to cause an increase in the value of the angle of 



