143 



the primary gut (archenteron) which in sea urchins is located at an angle to 

 the animal-vegetal axis. At the place of the vegetal pole a blastopore forms 

 (Figures 99, 100). The process of formation of an early gastrula in sea urchins 

 proceeds at a variable rate. In the sand dollars Echinarachnius parma and 

 Scaphechinus mirabilis, it takes just 8 hrs from the moment of hatching of the 

 blastula, while a slow rate of development, 2 days, has been reported for 

 Echinus esculentus (McBride, 1903). The late gastrula appears in 

 Echinarachnius and Scaphechinus mirabilis 12 hrs after hatching of the blas- 

 tula; a similar time has been reported for Echinostrephus molaris (Onoda, 

 1936). In other species of sea urchins the process of gastrulation is completed 

 within 14-19 hrs (Onoda, 1931, 1936, 1938; Fenaux and Fenaux, 1974), in 

 still others only after 23-32 hrs (Aiyar, 1936; Onoda, 1936, 1938; Kryuchkova, 

 1977). 



After the formation of the archenteron, the cells of the primary mesen- 

 chyme begin to accumulate at two sides of its base. Soon one can distinguish 

 a small calcareous granule in the center of each such cluster which gradually 

 acquires the shape of a triradiate spicule. From this moment, the formation 

 of the larval skeleton begins (Figure 100). The formation and growth of the 

 skeletal rays were thoroughly studied by Okazaki (1960, 1963). Simulta- 

 neously with the formation of the calcareous granules, the coelomic sac 

 separates from the upper end of the archenteron. This process is accompanied 

 by the expulsion of the cells of the secondary mesenchyme (Gustafson and 

 Wolpert, 1967). The coelomic sac soon divides into right and left parts. After 

 this, in the region of the future esophagus an invagination forms, which 

 gradually deepens and reaches up to the upper end of the archenteron, causing 

 the walls of the archenteron and ectoderm of the esophagus to rupture, which 

 leads to the formation of a through gut. From this moment the blastopore 

 assumes the function of an anus. Soon the region of the vegetal pole of the 

 gastrula flattens and the animal pole region inclines somewhat towards the 

 ftiture dorsal side (Figure 101). The larva, still entirely covered with cilia, 

 acquires the shape of a prism. This process has been studied in detail by 

 Onoda (1931) in Heliocidaris crassispina. 



Prism Stage 



Duration of this stage, corresponding to the dipleurula in sea stars, has 

 seldom been observed. 



Differentiation of the intestinal tube into the esophagus, stomach and 

 small intestine occurs in the prism stage. As a result of contraction of the 

 apical areas of the presumptive cells of the sphincters, two constrictions occur 

 in the digestive tract, which become the cardiac and pyloric sphincters of the 

 stomach (Burke and Chia, 1980). The regions forming the esophagus, stom- 

 ach and small intestine do not differ in intensity of cell proliferation; the total 



