III. 8 



CHEMICAL COMPOSITION 



143 



i. SO 4 . 



This is ordinarily provided by MgSO 4 and CaSO 4 ; when the 

 fertilized ova are placed in a solution in which MgCl 2 is sub- 

 stituted for it (as, for example, in 3 % NaCl + -07 % KC1 + 

 5 % MgCl 2 + CaHPO 4 + CaCO 3 ) then their development is 

 retarded from the blastula stage onwards, the embryos are small 

 and degenerate without reaching the Pluteus stage (Fig. 74 B). 

 The gut is straight instead of bent, and not divided into the 



r 



FIG. 74 B. 



a. Normal position of skeletal spicules in Sphaerechimts. b, d. Ab- 

 normal position and number after treatment with S0 4 -free medium. 

 c. Larva of Echinus from a S-free solution, e. Pluteus of Sphaerechinus 

 with three fenestrated skeletal arms, instead of two. Treated with 

 a S0 4 -free medium and replaced in sea-water. /. Normal Pluteus of 

 Sphaerechimis. (After Herbst, 1897 and 1904.) 



usual three parts ; in Sphaerechinus no mouth is formed, the gut 

 is evaginated (Exogastrala). The endoderm is very thick, the 

 cells dark and dense. 



The sulphuric acid radicle (sulph-ion) is thus necessary for the 

 proper development of the gut, and necessary from the very 

 beginning, for in embryos which have been kept in SO 4 -free 

 water up to the mesenchyme-blastula stage and then replaced 

 in sea- water the alimentary tract is still abnormal. 



Deprived of SO 4 , in fact, the gut remains radially symmetrical, 



