William J. Moenkhaus 33 



and vigorous at the time the eggs were taken. I think, therefore, that 

 the eggs of Menidia notata can be almost perfectly impregnated by Fun- 

 dulus heteroclitus. 



The character of impregnation is in striking contrast to that of the 

 reciprocal cross, in that practically all the eggs fertilized are normally 

 impregnated. There was an occasional dispermic and some polyspermic 

 eggs. This is true whether the per cent of eggs impregnated is small or 

 large. None of these dispermic or polyspermic eggs were isolated to 

 see how far they would develop, nor preserved for the study of their 

 internal character. This difference in the character of impregnation in 

 reciprocal crosses I have found nowhere so strongly marked in any 

 of the many other crosses I made among fishes. The dispermic condi- 

 tion of 50 per cent of the eggs when Fundulus is used as the female, 

 is regular and occurs in every experiment, so that this diffrence is a 

 constant one. 



At the time these experiments were made for the first time, I was 

 not aware that any crosses between so distantly related species of fishes 

 had been recorded. Subsequently I found that Appellof, 94, had made 

 an equally remarkable cross between two European species : Labrus 

 rupestris, female, and Gadus morhua, male. He says nothing about the 

 percentage of eggs impregnated except that " ein Anzahl " were found 

 in regular cleavage the following day, nor about the character of impreg- 

 nation — whether any of the eggs were dispermic or polyspermic in addi- 

 tion to the normally impregnated ones. Pfiiiger's experiment, 82, in 

 which he succeeded in impregnating the eggs of an anuran, Eana fusca, 

 with the sperm of a urodele, Triton alpestris and Tiiriton tgeniatus is in 

 some respects even more remarkable. However, he succeeded in obtain- 

 ing only polyspermic impregnation. Morgan, 94, succeeded in impreg- 

 nating the eggs of Asterias with Arbacia. Mathews, 02, repeated the 

 experiments and concludes that Morgan's impregnations were probably 

 a species of parthenogenesis consequent upon shaking the eggs and not a 

 true impregnation by the sperm of a sea urchin. My experience with 

 many othe? crosses between fishes as distantly related as Fundulus and 

 Menidia incline me to the belief that the normal impregnation of these 

 two classes of Echinoderms is perfectly possible. This remarkable ex- 

 periment deserves to be repeated with all possible precautions. 



V. Development. 

 1. Cleavage. 



a. Form of Cleavage. — The cleavage of the eggs normally impregnated 

 goes on in a perfectly normal manner. The eggs all pass regularly 

 3 



