EARLY EMBRYONIC PROCESSES IN MARINE EGGS 509 



between the two races as regards the spirality of the cleavage. The 

 cleavage is of the spiral type from the beginning both in the Mediterra- 

 nean and in the North Sea. This makes paragraph D (2) above ob- 

 solete and no further attempts will be needed to explain a phenome- 

 non, which does not exist. —However it is interesting to note that re- 

 cently W. Ludwig and H. W. Ludwig have reported physiological dif- 

 ferences in the regenerative potency of Hydroides norvegica from dif- 

 ferent localities, such as Naples, Plymouth and Cold Spring Harbor 

 (Ludwig, W. and H. W. Ludwig "Untersuchungen zur kompensatoris- 

 chen Regeneration an Hydroides norvegica" Roiix' Arch. Entwicklungs- 

 mech. 147, 259, 1954). It is also noteworthy that in another marine 

 Polychaete, OpJiryotrocJia puerilis, Bacci and La Greca have described 

 slight physiological and morphological differences between the races 

 inhabiting the North Sea and the Mediterranean respectively (Bacci, G. 

 and M. La Greca "Genetic and morphological evidence for subspecific 

 differences between Naj^les and Plymouth populations of Ophryotrocha 

 puerilis" Nature 111, 1115, 1953). Finally it should be mentioned that 

 Steinbock and Ausserhofer have found in a primitive fresh-water flat- 

 w^orm, Proritynchus stagnalis, two fundamentally different types of clea- 

 vage within the same species, one equal and orthoradial, one unequal 

 and spiral.* These findings seem to corroborate in one way or another 

 the general conclusions of this paper. 



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