148 EXTERNAL FACTORS III. 8 



eggs have been kept in sea- water, and such eggs develop, in 

 Mg-free water, to precisely the same extent as those which have 

 been fertilized in sea- water ; at this period, therefore, the egg 

 needs no external magnesium. The original store of this element 

 apparently suffices also for the first steps in the formation of the 

 gut; for the lack of it is felt equally in the later stages of its 

 differentiation and in the first moments of its development; the 

 alimentary canal is as abnormal in those larvae which have been 

 kept without magnesium only up to the time when the mesen- 

 chyme and archenteric plate arise as in those which remain in 

 the solution throughout. 



TABLE XIX 



Showing the retardation of development of Sea-urchin larvae 



deprived of magnesium. (After Herbst.) 



Two solutions were employed, mixed in various proportions; one (1) 

 has no Mg ; the other (2) contains Mg. 



Solution. Result after 24 hours. 



1 Nearly motionless ; archenteron formed. 



1 + 10 % of 2 Larger. 

 1+20% of 2 Larger still. 



1 + 30 % of 2 Larger ; gut constricted ; mouth formed. 

 1+40% of 2 

 1 + 50% of 2 



55 > 5 55 5) 



It is, in fact, only after the gastrula stage that magnesium is 

 absorbed. Eggs, blastulae and young gastrulae, reared in sea- 

 water develop far worse when placed in the Mg-free liquid 

 than do gastrulae. 



The formation of pigment and the contractility of muscles 

 remain unaffected by the absence of Mg. 



In Mg-free media the blastomeres of Asierias fall apart and 

 the adult medusae of Obelia degenerate. 



vi. Ca. 



The calcium salts present in normal sea- water are CaCO 3 , 

 Ca 2 SO 4 , CaHPO 4 , and Ca 3 (PO 4 ) 2 . 



When the carbonate only is absent the blastulae are crumpled 

 and opaque ; a few gastrulate but are markedly abnormal, the 

 ciliated ring being crumpled, the gut flattened on the oral side 

 and the skeleton absent (Fig. 74 A, g}. Should the skeleton 

 nave already been formed when the larvae are exposed it 



