EARLY EMBRYONIC PROCESSES IN MARINE EGGS 507 



of the production of morphogenetic substances, i.e., differentiation. The 

 former tend to be constant under various climatic conditions whereas 

 the latter follow exactly the differences of temperature, i.e., will proceed 

 slower in colder and quicker in warmer climate.* 



(2) This means that various stages of cleavage will not coincide in 

 all egg races with the same stages of morphogenetic processes. In colder 

 watei^s morphogenesis will proceed slower in relation to cleavage than 

 in warmer waters. Therefore a certain stage of differentiation which 

 in a race of a warmer region is reached, say in the 8-cell, or 16-cell stage, 

 will be reached in a race from a colder region perhaps only in the mo- 

 rula or blastula stage. 



(3) The relative time of differentiation defines also the mode of 

 determination of subsequent developmental processes in the various egg 

 races. A race with relatively early differentiation, i.e., an early produc- 

 tion and segregation of morphogenetic substances, will have a mosaic 

 type of determination, in which the fate of the blastomeres is fixed from 

 the beginning. In a race in which such morphogenetic processes start 

 relatively late and progress slowly, the development will be of the reg- 

 ulative type; i.e., the morphogenetic potency of the blastomeres is in 

 the beginning greater than their final role. 



(4) This difference may also determine the mode of cleavage as it 

 is generally assumed that "mosaic" eggs have a tendency to unequal 

 cleavage, whereas in regulative eggs the cleavage is usually equal (cf. e.g. 

 Lillie 1901, Chen and Pai 1949). The explanation is, of course, that in 

 the mosaic eggs there are invisible differences between the various re- 

 gions of the cytoplasm from the beginning, due to a precocious produc- 

 tion and segregation of morphogenetic substances and these cause dif- 

 ferences in the shape and size of blastomeres. In the regulative eggs 

 the cytoplasm is in the beginning more or less homogeneous and con- 

 sequently the blastomeres will be approximately equal. 



D. Interpretation and Generalization of the Case of 

 Hydroides Norvegica 



(1) The above theoretical considerations are in good accordance 

 with the findings about the mode of cleavage in Hydroides. The cleav- 

 age is equal in the North Sea, i.e., in a colder region, and unequal in 

 the Mediterranean, which is definitely warmer. On the basis of the fore- 

 going the explanation must be that in the Mediterranean race the pro- 

 cess of differentiation begins relatively early and the cytoplasm of the 

 eggs must be differentiated into heterogeneous regions before the cleav- 



• Findings of Koehler (1912) seem to indicate that the two sets of processes have differ- 

 ent temperature coefficients even within the same egg, which shows that they are of differ- 

 ent nature (Koehler, 1. cit. page 273). 



