BLASTULA, GASTRULA AND PLUTEUS III 



It was found many years ago by Pouchet and Chabry (1889) that 

 calcium is necessary for the formation of the pluteus skeleton. Develop- 

 ment would not take place in sea water without any calcium, since 

 the cells break apart (Herbst, 1900). They found that if i/ioth the 

 normal amount of Ca in sea water was replaced by Na, no skeleton 

 was formed. Loeb (1900a) found that Mg and CO3 ions are also 

 necessary for a normal skeleton. He could obtain plutei of Arbacia with 

 normal skeleton in a solution of: 95 cc. 5/8 n NaCl + i cc. 10/8 n 

 MgCla + I cc. 5/8 n KCl + 2 cc. 10/8 n CaClg + i cc. 1/8 n NagCOa. 

 Herbst, in a series of papers (1892-1904) studied the relation of the 

 composition of the sea water to the development of the pluteus. He 

 showed that SO4 is necessary for the development of the skeleton 

 (1904). See J. D. Robertson's review (1941). 



The development of the triradiate spicules in Echinus esculentus has 

 been studied by Woodland (1906, 1907). The spicule arises in an 

 early mesenchyme cell as a granule which becomes three-cornered. 



The digestive tract of the pluteus is J-shape consisting of a mouth 

 on the dorsal side between the two shorter arms, an oesophagus, 

 stomach, and intestine and ending in the anus between the two long 

 arms on the ventral side (Plate IV, Photographs 7, 8). 



There are large red pigment spots scattered irregularly over the 

 body and often more abundantly along the arms, especially at their 

 tips. They are sometimes irregular in shape, sometimes spherical 

 (E. B. H.). Each spot consists of 20 to 30 individual granules about 2 \x 

 in diameter. The pigment spot itself is variable in size, an average 

 spherical one measuring about 7 [/ in diameter. The red pigment is 

 echinochrome, the same that is found in the chromatophores of the 

 egg, having the same composition and absorption spectrum (Ball and 

 Cooper, 1949). The granules swell in distilled water, like the chroma- 

 tophores of the egg, having similar osmotic properties (E. B. H.). The 

 permeability value has not been studied, and it would be interesting 

 to compare this with the permeability value of the chromatophores as 

 given by D. L. Harris (1943). The gradual decrease in number of the 

 chromatophores and appearance of the pigment spots in the very early 

 pluteus stage following the prism stage might also prove to be an inter- 

 esting study. 



Photographs of a well developed pluteus are shown on Plate IV, 

 Photographs 7-9. These are anal (ventral) and oral (dorsal) views 

 of the same animal, and a side view. 



When photographed with ultraviolet light (2537 A), certain regions 

 appear much darker than others (E. B. Harvey and Lavin, 1951a). 



