J. R. GREGG 237 



and synchronously with control embryos.* Small differences be- 

 tween advanced blastulae, however, might be difficult to detect. 



2. Hybrid embryos do not begin to gastrulate until controls 

 are in Shumway Stage 10+-11~. We shall express this by saying 

 that hybrids begin to gastrulate only when they are in stage 

 H10+-H11 .f At 18° C this represents a developmental delay 

 of about 3 to 4 hours; but, in appearance, hybrids at this stage 

 are exactly like controls in Stage 10. 



3. Once formed, the blastopore of a hybrid embryo does not 

 enlarge appreciably. (Some rare exceptions to this rule are de- 

 scribed in Moore's (1946) paper.) This should not be read as 

 stating that all gastrular movements cease at this stage, for this 

 is not true, as we shall see. 



4. When hybrids are in Stage H17 they develop functional 

 external cilia, which rotate them slowly within their tighter-than- 

 normal vitelline membranes. Control embryos develop cilia when 

 they are in Stage 15. 



5. By the time they are in Stage H20-H21, hybrid embryos 

 have swollen until their diameters are some 20% greater than 

 their diameters at Stage HIO, and their animal hemispheres have 

 become curiously pitted. Possible mechanisms of swelling were 

 discussed by Moore and also by Gregg ( 1948 ) , but none of their 

 suggestions (all of which postulated hypemormal intrablas- 

 tocoelic hydrostatic pressure ) has yet been put to test. 



6. When hybrid embryos are in Stage H20-H21, their blas- 

 tocoel roofs usually rupture, possibly as the result of intolerable 

 internal pressure, and they shrink to their original sizes, approxi- 

 mately. 



7. Most hybrids die by progressive cytolysis when they are 

 in Stage H21 (about 160 hours after fertilization, at 18° C). It 



* We shall use "control" or "control embryo" in accordance with the 

 rule that re is a control for y provided that the following conditions are 

 satisfied: (i) x is a member of R. pipiens; (ii) y is a hybrid; (iii) x and y 

 have the same female parent, and have developed under similar environ- 

 mental conditions for the same length of time. 



f In general, we shall say that hybrids are in Stage H^ if their controls 

 are in Shumway Stage s. 



