150 EXTERNAL FACTORS III. 8 



cohere, and separate as fast as division takes place, swimming 

 about independently for a time, and then dying. The same 

 phenomenon is witnessed when later stages are placed in such 

 a calcium-free mixture. 



On removal to sea-water division continues without separation, 

 and should the egg membrane still be intact all the cells unite 

 and a whole larva is formed. Even should the egg membrane 

 be lost, a reunion of the cells is always possible so long as they 

 remain in contact with one another. The separation is due to 

 a change in the surface-tension of the cells; a visible change 

 takes place, in fact, in the superficial layer which covers and 

 unites the blastomeres; it becomes ill-defined and radially 

 striated. 



The lack of calcium also affects the rate of development, and 

 causes shrinkage, but leaves karyokinesis, ciliary motion, and 

 pigment formation unaltered. 



Calcium is not replaceable by magnesium, strontium, or 

 barium. 



vii. CO 3 . 



As has just been pointed out, calcium carbonate is necessary 

 for the due formation of the skeleton, although a beginning 

 may be made without it. 1 



Whether the crumpling of the larva, due to diminution of 

 internal osmotic pressure, which is observed in the absence of 

 calcium carbonate is attributable to the lack of CO 3 or the lack 

 of the hydroxyl ion is difficult to determine, since, as Herbst 

 points out, a carbonate necessarily introduces OH, while the 

 latter can convert into carbonates the CO 2 of the atmosphere and 

 of respiration. 



viii. OH. 



The alkalinity of the sea-water reckoned by the number of 

 free hydroxyl ions is provided by the calcium carbonate and 

 calcium hydrogenphosphate. By the omission of these a solution 

 neutral to litmus may be obtained in which the ova give rise 

 to thick-walled, opaque blastulae with granular contents, ciliated 



1 CaC0 3 is necessary for the formation of the skeleton of the larva of 

 the sponge Sycandm setosn (0. Maas, S.-B. Ges. Moryih. Phys. Munchen, 

 xx, 1905). In a medium devoid of all calciiun salts the Amphiblastulae 

 fall to pieces. 



