MODES OF INHERITANCE IN HYBRIDS 469 



on the idea that the effects of foreign sperm are always injuri- 

 ous, resulting in retardation, can explain both of these equally 

 obvious results. Another point of rather general import is also 

 illustrated by this chart. It often happens in cases where hybrid 

 strains go faster or slower for the first few days of development, 

 that, in so far as the general external evidences of degrees of 

 development may be relied on as criteria, hybrid and pure strains 

 reach a point at which they are for a short time on a par. This 

 is well illustrated by the figures showing the three strains of F. 

 diaphanus eggs at the end of one week (figs. 18, 19, 20). After 

 this time acceleration or retardation is again evident, usually 

 up to the period of hatching. I am unable at present to offer 

 any suggestion as to the conditions underlying this apparent 

 interruption in the developmental rhythms of these strains. 



In all cases in which hybrids successfully weather the period 

 of gastrulation and enter upon the period of embryo formation, 

 further success seems to be conditioned by the ability or in- 

 ability to establish nutritive relations with the yolk. In cases 

 where hybrids differentiate a circulatory system but fail to com- 

 plete the assimilation of yolk, the difificulty is evidently due to 

 lack of balance between the rate of embryonic differentiation 

 and that of yolk digestion, as will be shown later. Although in 

 the hybrids under discussion (F. diaphanus ? X F. majalis cf 

 and F. diaphanus cf X F. majalis 9 ) a great many of the em- 

 bryos become abnormal for these reasons, it must not be for- 

 gotten that in every experiment a large proportion of them sur- 

 mount the difficulties of yolk assimilation and produce normal 

 vigorous larvae, some of which, at least in the F. heteroclitus 

 egg hybrids, are more viable and grow faster than either of the 

 pure bred strains. It is difficult to imagine what factors under- 

 lie this wide range of success of individual hybrids of the same 

 parentage. It has been suggested that the best results are ob- 

 tained when both germinal products are at the optimum state 

 of maturity, and that if either one or the other germ cell be at 

 all under-ripe or stale the result is sub-optimal development of 

 varying degrees depending on the degree of over- or under-ripe- 

 ness of egg or sperm. A more nearly probable explanation, how- 



