548 H • H . Newma?7 



IV SUMMARY OF DATA 



1 The volume of F. majalis eggs is on the average more than 

 twice that of F. heteroditus. The rate of development is rather 

 closely proportional to the mass of the eggs, the smaller egg devel- 

 oping nearly twice as fast as the larger egg. 



2 The yolk of F. majalis eggs is much more yellow and opaque 

 than that of F. heteroditus. This optical difference undoubtedly 

 indicates a much more deep seated difference in chemical composi- 

 tion. 



3 A far larger percentage of eggs are fertile to sperm of their 

 own species than to that of foreign species. 



4 The rate of early cleavage is not measurably altered by the 

 introduction into the egg of sperm belonging to a more slowly or 

 more rapidly developing species. 



5 The earliest measurable effect of foreign sperm in hastening 

 or retarding development is seen in the egg of F. heteroditus. At 

 periods ranging from fourteen to twenty hours the blastodisc of 

 H pure shows a measurably greater diameter than that of H 

 hybrid. In the eggs of F. majalis this difference is not seen for an 

 average of about six hours later, but it is just as marked when it 

 appears. 



6 In general the development of F. heteroditus eggs is retarded 

 by the introduction of F. majalis sperm, while that of F. majalis 

 eggs is accelerated by the introduction of F. heteroditus sperm. 

 This acceleration or retardation is not permanent for either of the 

 hybrid strains. The more fortunate of the H hybrids, although 

 retarded forthe first eight or ten days, at and subsequent to hatching 

 are somewhat larger, have a more rapid and more efficient circula- 

 tion, are more active in their movements, show a greater resistance 

 to lack of oxygen and the presence of carbon dioxide, and live 

 much longer in captivity, than do any of the H pures. The M 

 hybrids, on the other hand, develop more rapidly than do the M 

 pures for a period of from seven to ten days, but after that they 

 gradually cease to grow, attain a size only about half that of the 

 M pures at hatching (but about equal to that of the H pures at 

 the same period) and never succeed in hatching because of their 



