HYBRIDS BETWEEN FUNDULUS AND MACKEREL 401 



The mackerel egg gives very poor results when inseminated 

 with foreign sperm of any of the numerous species tried. A very 

 large percentage of eggs develop, but many cleave quite irregu- 

 larly. A somewhat smaller percentage proceed normally or 

 nearly so up to gastrulation and then die. I have never succeeded 

 in raising mackerel eggs inseminated with the sperm of any foreign 

 species up to the point of embryonic differentiation, and hence 

 no chromatophores are formed. This study of heredity is there- 

 fore confined to one of the two reciprocal crosses: that produced 

 from the Fundulus egg inseminated with mackerel sperm. 

 The striking difference between the developmental success of the 

 reciprocal crosses is not at all an uncommon phenomenon. 

 I described in a former paper numerous other cases of the same 

 sort (Newman, '15). The explanation seems to be that the 

 mackerel egg is extremely sensitive to foreign sperm material 

 and fails to tolerate it beyond the cleavage period. 



A detailed account of a single typical experiment in hetero- 

 genic hybridization will serve to bring out the essential facts. 



A typical hybridization experiment between Fimdulus 9 X 



mackerel d^ 



The following account gives the details of five experiments 

 performed on June 12 at the Woods Hole fish traps. An un- 

 usually fine lot of female Fundulus were segregated from males 

 and taken in a large aquarium in order to have the best possible 

 conditions. A large female was chosen, the eggs stripped into a 

 bacteria dish and inseminated with milt from a vigorous male 

 mackerel just taken from the traps. After about ten minutes 

 the excess of milt was washed out carefuUj^ to avoid contamina- 

 tion of the water. Four other exactly similar experiments were 

 performed using other males and females. This one experi- 

 ment here described gave a somewhat wider range of types than 

 the others, but all gave substantially the same results as far as 

 fundamental matters are concerned. After about five hours the 

 eggs in all experiments were examined and all unfertilized eggs 

 removed. In the five experiments the following percentages of 



