STRUCTURAL CHANGES AT FERTILIZATION 99 



efficacious as the latter. When equal quantities of sea water and 

 of isotonic potassium chloride, oxalate, or citrate are added to 

 fertilized eggs of Arbacia punctulata, eleven, but not less than 

 eleven, minutes after fertilization (i.e. at the monaster stage), the 

 echinochrome granules explode and echinochrome is found in 

 the surrounding sea water (Churney & Moser, 1940). The eggs 

 subsequently cleave in the usual way. Heilbrunn says that this 

 experiment demonstrates the release by potassium of calcium, 

 previously bound in the cortex. These observations may have 

 some bearing on the 'clotting reaction', as Donnellon (1938) 

 observed that, during the clotting of sea-urchin perivisceral fluid, 

 the red granules in the red amoebocytes and the colourless ones 

 in the white amoebocytes explode. Isotonic KCl has the same 

 eff"ect (cf. exploding platelets in mammalian blood-clotting.) 



(5) Acid production, which was not inhibited by iodoacetate 

 to the same extent as Oo uptake. The addition of papain, in- 

 stead of CaClg, also induced acid formation. 



These striking observations indicate once more the importance 

 of calcium in the metabolism of eggs. Moreover, they confirm, as 

 Heilbrunn has so often insisted, that changes in the distribution of 

 calcium have profound effects on protoplasmic structure. 



The ejfects of changes in the external environment on the cortex. 

 An interesting contribution to our knowledge of the cyclical 

 changes in cortical structure which occur after fertilization and 

 parthenogenetic activation was made by Herlant in 1920, using the 

 eggs of Paracentrotus {lividus}) and Sphaerechinus (granularis 

 (Lamarck)?). Herlant examined two things: the incidence of 

 plasmolysis (shrinking) and cytolysis in eggs exposed to various 

 hypertonic solutions ; and the variation in the susceptibility of eggs 

 to a number of cytolytic agents, at various times after fertilization. 

 As Herlant's paper is extremely long and does not contain a sum- 

 mary, its contents are summarised below: 



(i) Fertilized sea-urchin eggs undergo cyclical variations in 

 their susceptibility to hypertonic sea water (100 ml. s.w. + 20-25 

 ml. 2-5M-NaCl), as indicated by plasmolysis. Unfertilized eggs 

 plasmolyse, the surface of the egg becoming wrinkled, but this 

 characteristic progressively disappears after fertilization, until 

 there is a zero plasmolysis from 5-25 minutes after fertilization. 

 From 25-70 minutes, plasmolysis re-occurs, becoming maximal 



