J. R. SHAVER 269 



species could be shifted in accordance with a centripetal shifting 

 of the cortical cytoplasm. Invagination started at the point where 

 the cortical cytoplasm was thickest. This observation seems to 

 agree with the earlier ones of Runnstrom ( 1928b ) who described 

 an area in the cortex of the vegetal half of sea urchin eggs which, 

 when followed by dark field illumination, became localized in 

 cells which were invaginated to form the archenteric walls of the 

 embryo. 



Later studies (Motomura, 1941; Runnstrom, 1949; Endo, 1952) 

 relate the release of cortical granules to the formation of the fer- 

 tilization membrane. Release of material to the cell interior may 

 well play an important part in activating preexisting patterns for 

 differentiation in the egg (Runnstrom, 1952). Suggestions as to 

 the nature of this mechanism have been made by Heilbrunn 

 (1952), who emphasizes the colloidal changes produced in the 

 cell interior by the release of calcium from the cortex. Perhaps a 

 more specific type of mechanism that could be set in motion by 

 the release of materials inward from cortical granules (or other 

 cortical areas ) at fertilization is one suggested by Tyler ( 1947 ) , 

 whereby differentiation is envisaged as the result of interactions 

 of substances similar to antigen-antibody reactions. As Tyler re- 

 marks ( 1947 ) : "To account for these differences in location and 

 amount ( of specific antigens of the organism ) we must again fall 

 back on initial regional differences in the cytoplasm of the un- 

 cleaved egg." The application of newer techniques for preparing 

 cytoplasmic fractions of greater homogeneity ( cf . Hogeboom and 

 Kuff, 1955 ) than heretofore, against which antisera could be made 

 for testing developmental effects, might be a profitable means of 

 testing this interesting idea. 



Although not related directly to the differentiation of the em- 

 bryo, some studies on the effects of granular fractions of embry- 

 onic and adult tissues of vertebrates and of sea urchins, on the 

 fertilization and cleavage processes, are of physiological interest. 

 Runnstrom et at ( 1954 ) have observed the effect of an extract of 

 a granular fraction, and of a supernatant fluid, prepared from sea 

 urchin eggs, on the fertilization of eggs of Arhacia lixiila. All the 

 control eggs apparently were of the tight membrane type, and 



