508 EIGHTH PACIFIC SCIENCE CONGRESS 



age begins, so that the blastomeres will be of different size. In the North 

 Sea race, on the other hand, the egg cytoplasm must be practically 

 homogeneous during the first stages of cleavage and consequently the 

 blastomeres will be more or less equal. 



(2) The further fact that the cleavage of Hydroides is orthoradial 

 in the Mediterranean race and spiral in the North Sea race may be per- 

 haps also explained by differences of temperature. Knowing that the 

 two types of cleavage depend to a certain extent on differences in the 

 cohesion of the blastomeres (cf. Gray 1931), and knowing that calcium 

 plays a decisive role in this cohesion (cf. Herbst 1900), one may specu- 

 late if the difference in the cleavage pattern is due to differences in the 

 availability of calcium in the waters of the two regions, caused by dif- 

 ferences of temperature. However, this trend of thought has its serious 

 obstacles and for the time being no satisfactory explanation can be of- 

 fered to account for the phenomenon. 



(3) The explanation of the differences in the mode of cleavage of 

 Hydroides can be generalized and a tentative prediction put forward 

 concerning physiological egg races. It can be predicted that marine 

 species occupying large geographical areas, within which there are dif- 

 ferences of climate, probably have physiological egg races differing from 

 each other in the type of the determination of their developmental 

 processes and their pattern of cleavage. In colder regions the eggs 

 of such species should be of the regulative type and their cleavage 

 equal. In warmer regions one should expect to find egg races of the 

 same species with a "mosaic" character and unequal cleavage pattern. 

 The same difference should be found between eggs of closely related 

 species occupying different geographical areas, in which the climate 

 is different. —These predictions could be verified, or disproved by 

 further investigations on a broad comparative zoogeographical basis. 



ADDENDUM AT PROOF-READING 

 Since this paper was submitted I have had an opportunity to study 

 the mode of cleavage of Hydroides nonegica in the Mediterranean (cf. 

 Wolsky, A. "Mode of cleavage of the eggs of Hydroides norvegica in the 

 Mediterranean," Nature 176, 75, 1955). It turned out that the dif- 

 ference between the eggs from the North Sea and from the Mediterra- 

 nean is not so sharp as assumed on the basis of Soulier's categorical state- 

 ment (Soulier 1898, page 1662) "chez Hydroides .... la segmentation 

 est inegal au debut". Measurements have shown that the cleavage is 

 indeed somewhat more unequal in the Mediterranean than in the North 

 Sea, but it was impossible to decide whether this difference is statisti- 

 cally significant. On the other hand, definitely no difference was found 



