506 EIGHTH PACIFIC SCIENCE CONGRESS 



salinity has the same effect as lower temperature (cf. Heuts 1947b, Koch 

 and Heuts 1943). 



(2) The most immediate adaptive response of the organisms to 

 these factors is the adaptation of the tolerance range and optimum of 

 their eggs to the conditions existing in their environment. Eggs prod- 

 uced in warmer waters tolerate higher temperatures and have a higher 

 optimum temperature for their normal development than eggs from 

 colder waters. This was shown both for egg races within the same spe- 

 cies and for eggs of closely related species inhabiting different regions 



(cf. Runnstrom 1927, 1929, 1936, Moore 1939, 1942a, b, 1944, 1949). 

 Similar differences were found between egg races from localities of 

 different salinity in respect to adaptation to osmotic pressure (cf. Heuts 

 1947b, 1949). 



(3) Certain physiological properties of eggs, in particular the rate 

 of cleavage, are adapted to temperature (and salinity) in a regulative- 

 compensatory manner. That is, the adaptation tends to minimize the 

 effect of extreme environmental influence and to establish a uniform 

 physiological standard under different conditions. Egg races from colder 

 waters have the same rate of cleavage at the average temperature of 

 their normal environment as egg races from warmer waters at the higher 

 average temperature of their normal habitat (cf. Munro Fox 1936, Moore 

 1939, 1942a, 1949). 



(4) Certain other physiological-biochemical properties of eggs, no- 

 tably the rate of the production of morphogenetic or "organ-forming" 

 substances (i.e. the beginning of differentiation which is often called 

 "chemo-differentiation") do not show such regulative-compensatory 

 adaptation. On the contrary, this seems to be influenced directly by tem- 

 perature (and salinity), so that morphological differentiation proceeds 

 quicker in warm regions (or regions of low salinity) and slower in cold 

 regions (or regions of high salinity). In consequence there may arise 

 some morphological differences between adult specimens of the same 

 species, collected from different localities differing in their average tem- 

 perature and/or salinity, which can be reproduced artificially in a lab- 

 oratory by controlling temperature and salinity during eynhryonic de- 

 velopment. This has been shown for example for the so-called meristic 

 characters of fishes, such as number of vertebrae, number of fin-rays, 

 etc. (cf. Heuts 1946, 1947a, b, 1948, Hubbs 1922, 1926). 



C. Some Theoretical Considerations Concerning the Correlation 

 BETWEEN Cleavage and Differentiation 

 (1) From the foregoing it seems that in egg races the processes of 

 cleavage are adapted to temperature in another way than the processes 



