270 EARLY ECHINODERM DEVELOPMENT 



the principal effect of the granule extract was to enable normal 

 membranes to be elevated at fertilization. A later eflFect of this 

 fraction was to counteract the tendency toward radialization seen 

 in the control larvae. The authors imply that the normal balance 

 between ventral and dorsal tendencies in sea urchin development 

 might be maintained by substances released from granules within 

 the egg. The supernatant Huid contained a jelly-precipitating fac- 

 tor which is compared in its physiological role with egg antifer- 

 tilizin (Tyler, 1940). 



Shaver ( 1953 ) studied the effects of various fractions of em- 

 bryonic and adult tissues of the frog in initiating cleavage when 

 injected into unfertilized frog eggs. Large granule fractions ap- 

 peared to be the most effective in this regard. Active fractions 

 were not obtained until just before gastrulation commenced. It 

 was suggested that a possible mechanism of action of the granules 

 in initiating cleavage might be connected with a protoplasmic 

 clotting system similar to the one postulated by Heilbrunn (1952). 



These studies, as well as others made in the period before the 

 importance of the homogenization medium for the preservation 

 of structure of intracellular cytoplasmic particulates was realized, 

 would appear to suffer from lack of homogeneity of the fractions 

 being tested. Runnstrom et al. (1954) used isotonic sodium chlo- 

 ride (or in some cases sodium and calcium chloride) as the ho- 

 mogenization medium, and Shaver ( 1953 ) employed a dilute 

 phosphate buffer. It has been pointed out (Schneider and Hoge- 

 boom, 1950 ) that the use of electrolyte solutions for homogeniza- 

 tion of tissues results in changes of the particles that may grossly 

 alter the distribution of substances in the fractions subsequently 

 isolated from the homogenate by centrifugation. 



Gradients and Cytoplasmic Particles in Early 

 Sea Urchin Differentiation 



Atomistic theories were certainly among the first and appear 

 to be among the most recurrent of attempts to explain cell and 

 tissue differentiation. From the invisible "pangens" of deVries to 

 the "plasmagenes" of more recent hypotheses, the intervention of 

 cytoplasmic particles or centers of one kind or another has often 



