III. 8 



CHEMICAL COMPOSITION 



149 



becomes dissolved. When the sulphate only is omitted (mag- 

 nesium sulphate being present) development is still inferior to 

 the normal, inferior even to development in the presence of 

 CaSO 4 but in the absence of MgSO 4 ; CaSO 4 may be replaced 

 by CaCl 2 . It seems, therefore, that the sulphate is necessary as 

 a calcium salt. 



FIG. 75. a-c. Separation of the blastomeres of Echinus microtiibercn - 

 lulus in a medium containing NaCl, 3-07%, KC1, 0-08%, MgSo 4 , 0-66%, 

 MgHP0 4 and FeC0 3 , but no Ca. Note the radially striate border, which 

 is the altered uniting membrane, d. Blastula disintegrating in the same 

 medium. (After Herbst, 1900.) 



If only the phosphate (Ca 3 (PO 4 ) 2 ) is present the egg- dies 

 during segmentation, though, if the other phosphorus compound 

 is substituted for it, the effect is the same as when the carbonate 

 alone is omitted. 



Should, however, all calcium salts be removed the result is 

 more serious still (Fig. 75). The blastomeres are unable to 



