100 FERTILIZATION 



after 45 minutes and remaining so until 75 minutes after ferti- 

 lization, when mitosis begins (17° C). During mitosis there is 

 no plasmolysis. This description of the reactions of fertilized 

 sea-urchin eggs to hypertonic sea water is somewhat surprising 

 and does not agree with the descriptions of Hobson (ig^^a) and 

 Monroy & Montalenti (1947); the experiments, which are 

 technically simple, could be repeated with advantage. Inter- 

 pretation, however, is not easy, because of the delay between 

 treatment and the onset of symptoms. 



(2) In grossly hypertonic sea water (60 ml. s.w. + 40 rnl. 

 2-5M-NaCl), the eggs either plasmolyse or cytolyse, the in- 

 cidence of the former being inversely proportional to that of the 

 latter. 



(3) If it is assumed, as Herlant does, that changes in the ex- 

 ternal environment directly and exclusively affect the properties 

 of the cortex, NaCl- and KCl-enriched sea water increase the 

 permeability of the cortex, while CaClg and MgClg have the 

 reverse effect. According to Herlant, an increase in the incidence 

 of plasmolysis implies a decrease in permeability to salts, it being 

 assumed that permeability to water is unaffected by the experi- 

 mental treatment; conversely, that an increase in cytolysis is 

 caused by an increase in permeability to ions. Tracer work will 

 clarify these questions in the next few years. 



(4) Herlant also investigated the effects of various agents on 

 the incidence of cytolysis in fertilized and unfertilized sea- 

 urchin egg suspensions; his results are summarised in Fig. 19. 



Herlant's experiments show without doubt that fertilization 

 induces a series of cyclical changes in the properties of the cortex.* 

 For example, the low resistance of the egg to digitonin in the in- 

 terval 10-35 minutes after fertilization suggests that, just before 

 this period, cholesterol or a cholesterol-like compound becomes 

 more accessible to the digitonin molecule, at the cell surface. These 

 and other results which may bear on the same subject are sum- 

 marised in Tabic 16. When examining this table, it must be re- 

 membered that, with the possible exception of those of Mitchison 

 & Swann (1954a), the methods for investigating changes in the 



* Herlant was not, of course, the first person to make this discovery, as a 

 perusal of R. S. Lillie's papers or Needham's Cheviical Embryology shows. But 

 his work was far more comprehensive than anything done before, or, probably 

 since, (iQSS)- 



