The Process of Heredity 52 1 



forty specimens examined four showed the feeble beginnings of 

 a heart-beat, which was slow and irregular and at the rates of 26, 

 36, 44 and 42, an average of 37 beats per minute. These four 

 specimens and some others were somewhat larger in size than the 

 rest and showed a slight differentiation of the primary brain 

 vesicles. The optic vesicles were well cupped and the lens 

 spherical in all, but there was marked variation in the degree to 

 which the heads of the various embryos had developed. The 

 auditory vesicles were in all well defined. There were many 

 somites, usually over twenty. In none of the specimens was 

 there any pigmentation either on the body or on the yolk. In 

 none had quickening occurred. 



The M hybrids were at this time well in advance of the M pures. 

 The average condition is figured in the table {m^). The head 

 region of the majority of the M hybrids showed a condition even 

 more advanced than that of the other hybrid strain (Aj). There 

 was a marked outlobing of the inter-brain. The optic vesicles 

 were well cupped, the lens spherical and embedded in the cup. 

 Auditory vesicles were well defined. There were fifteen somites, 

 on the average (Plate II, m^). 



The M pures were less advanced, showing a less well developed 

 head region, and an average of about nine somites. No pig- 

 mentation present on either of the majalis strains (Plate III, M^). 



Stage 6 (80 hours) : During this rather short daylight period of 

 six hours, from 2 to 8 p.m., on a warm day, a considerable change 

 in the interrelationships of the four strains was apparent. The 

 H pures had advanced slightly. The heart-beat had increased 

 to an average of no beats per minute in ten specimens. The 

 pigmentation was considerably heavier and the size of the embryos 

 had increased noticeably (Plate III, H6). 



In the H hybrids, however, a remarkably rapid change had 

 occurred. The establishment of a heart-beat and a circulation, 

 early in the afternoon, had caused a very rapid advance and at 

 this time the H hybrids were nearly as well advanced as the H 

 pures. A few specimens were quite as advanced as any of the H 

 pures. The rates of heart-beat in ten examined were: 100,96, 

 90, 88, 90, 98, 92, 99, 90, 80. Average 92+. No pigmentation 



