44 POLARITY AND SYMMETRY; 



particles of the same sign would be concentrated at each of 

 the poles. This migration of the particles was assumed to be 

 caused by an electric field, due to differential penetration of 

 certain ions (potassium ions playing a major role) from the 

 environment into the egg. A difference in permeability of the 

 egg surface near the animal pole would be responsible for this 

 penetration. Later investigations, however, have shown that 

 "bipolar differentiation" is due not to segregation of colloid 

 particles of opposite signs in the cytoplasm, but rather to 

 shifting of the yolk of the egg. This material, consisting of 

 fat- and protein globules, now assumes its final position with 

 regard to the egg axis (Raven, 1938b). The acid yolk proteins 

 accumulate at the vegetative pole ; their disappearance from the 

 animal side causes the pH there to shift in the direction of 

 alkalinity. 



Gradient systems also play an important role in the devel- 

 opment of sea urchin eggs. Cleavage in these eggs is very 

 regular. It results in a blastula, a vesicle with a wall consisting 

 of a single layer of cells with cilia on their outer surfaces. In 

 the next stage, the wall of the blastula invaginates at the 

 vegetative pole, thereby forming the archenteron of an embryo 

 called a gastrula. At the animal pole the gastrula possesses a 

 long tuft of cilia (Fig. 15a). The further development of a 

 gastrula into a pluteus larva involves the growth of long 

 "arms", and the secretion of a skeleton of calcareous rods by 

 cells originating from the vegetative wall of the blastula. More- 

 over, the larva is now surrounded by a band of cilia which also 

 extends over part of the arms (Fig. 15b). 



Driesch has shown, as mentioned above (p. 28), that sea 

 urchin blastomeres isolated at the 4-cell stage can still develop 

 into a complete larva. This stimulated several investigators, in 

 particular Horstadius (since 1931) to study the developmental 

 potencies of blastomeres and groups of blastomeres, isolated 

 at later stages. In these investigations, the potencies of the 

 egg material were shown to change gradually from the animal 

 towards the vegetative pole. Isolated animal halves of a germ 

 will produce a larva with a large apical tuft, but without gut 

 and skeleton; isolated vegetative halves will grow into a larva 



