142 Papers from the Marine Biological Laboratory at Tortugas. 



HiPPONOE 9 X CiDARISC?: 



Hipponoe eggs were fertilized with Cidaris sperm 3 hours after their 

 removal from the ovary. No fertilization membrane was formed. The 

 fertilization-cleavage period was characteristic of the egg; segmentation 

 ver}^ irregular; larvs all of abnormal form. 



Hipponoe 9 X OphigcomacT*: 



Hipponoe eggs were fertilized with Ophioconia sperm 3 hours after 

 their removal from the ovary. No fertilization membrane was formed. 

 The fertilization-cleavage period was characteristic of the egg. All larvae 

 abnormal. 



Hipponoe 9 X PentaceroscJ^: 



Hipponoe eggs were fertilized with Pentaceros sperm 3 hours after 

 their removal from the ovary. No fertilization membrane was formed. 

 The fertilization-cleavage period was characteristic of the egg; the 

 segmentation and larvse irregular. 



THE EXPERIMENTAL CONTROL OF DOMINANCE. 



One of the most interesting and important problems connected 

 with the results of Echinoderm hybridization is the determination of 

 the factors influencing the appearance of maternal or of paternal char- 

 acters in the hybrid embryo. 



We have seen that Vernon (1898, 1900) showed a seasonal variation 

 and tried to show that the characters of the hybrid pluteus are dependent 

 on the relative ripeness of the sperm used in the crosses. Doncaster 

 (1904) concluded, as a result of his experiments, that the temperature 

 of the water in which the embryos developed is the determining factor, 

 Herbst (1906a, 19066, 1907), in an extended and able series of papers, 

 expressed his conviction that while temperature was a contributing 

 factor it was not the only one, and showed that the dominance might 

 be swung toward the maternal side by a combination of artificial par- 

 thenogenesis and fertilization; the actual cause of this displacement 

 lying in the preponderance of maternal nuclear material arising from 

 the application of this method. 



My own investigations have shed further light upon this unknown 

 factor. My experimental evidence shows that while the theories of the 

 investigators mentioned are correct, they do not present the whole 

 truth. My material has been especially fortunate in that I have obtained 

 a dominance of one species over another in both crosses. That is, when 

 the fertilization was made in normal sea-water Hipponoe characters 

 were dominant. By changing the conditions in which fertilizations 

 were made I have been able to change this dominance and to show that 

 the actual factor determining the dominance is directly concerned with 

 differences of season and of temperature, but that these two factors are 

 simply contributory. 



In my experiments, the factor determining the dominance is the vari- 

 ation in alkalinity of the sea-water in which the embryos develop. This 

 variation is probably dependent on season and temperature. 



