The Process of Heredity 519 



this condition since it was sufficiently obvious without measure- 

 ment (Plate II, Hz and hz). 



In M hybrids ocular micrometer measurements of the. diameters 

 of twenty blastodiscs were as follows: 21, 20, 23, 19, 21, 21, 19, 

 23, 18, 20, 20, 22, 21, 19, 17, 20, 21, 20, 22, 18. Average 20.25 mm. 

 (Plate II, wz). 



■ Twenty M pures gave 18, 16, 16, 17, 18, 16, 17, 17, 17, 18, 18, 

 16, 17, 18, 17, 16, 18, 16, 18, 17. Average 17 mm. (Plate II, Mz). 



The M pure and M hybrid embryos seemed to be about eight 

 hours behind the H pure and H hybrid embryos respectively. 

 The M hybrids showed a distinct and measurable advantage over 

 the M pures of the same age. According to measurements, then, 

 the two pure strains show" the extreme differences in time rate of 

 development, while both of the reciprocal crosses show inter- 

 mediate condition. 



Stage 3 (48 hours) : Nearly all of the H pures showed the con- 

 dition figured in Plate II {H^). The optic vesicles were large 

 and showed the cavity plainly. Three or four mesoblastic somites 

 were visible on practically all of the specimens examined. Out 

 of fifty-five specimens examined two were noticeably retarded, 

 showing much smaller optic vesicles without a visible cavity, 

 and no mesoblastic somites. 



The H hybrids showed a wider range of variabiHty. Of the 

 forty-seven examined seven or eight presented a retarded condition 

 similar to that of the two retarded specimens of H pure. Six 

 showed a condition stilLless advanced, resembling (mj), but with 

 the blastopore closed. No somites were visible on any of the 

 specimens. On the whole it was an easy matter to determine 

 that the H pures were considerably in advance of the H hybrids. 



With regard to the M pures and the M hybrids it was a matter 

 of considerable difficulty to determine which of the two strains was 

 in the lead (Plate II, mj and Mj). A fairly large per cent of the 

 M hybrids, however, showed a condition more advanced than any 

 M pures. One might be justified, then, in claiming that the hybrid 

 strain was in advance of the pure. Other series, as will be seen, 

 showed a much more marked difference betw^een these two strains 

 at the same stage of development. 



