Mniihraiif Fonnatiofi 367 



SUMMARY 



The essential points brought out in the preceding pages may 

 be summarized as follows: 



The action of acids in producing membranes on unfertilized 

 sea-urchin eggs is due to their combination with some substance 

 in the egg but the membrane is not the product of this combina- 

 tion. 



In composition the membrane is probably an albuminoid. It 

 is not present as such before fertilization. 



The essential condition for its formation is an increased per- 

 meability of the egg surface for a membrane substance which 

 passes out and hardens to the membrane in contact with sea- 

 water (a secretion). Double membranes may be explained on the 

 above theory. 



Several types of membrane may be produced under different 

 conditions and it is probable that the secretion of the membrane 

 substance always takes place although it ma^ remain close to 

 the egg surface. 



IV. MIGRATION OF THE PIGMENT GRANULES OF ARBACIA EGGS 



The second visible change which takes place after fertilization 

 in Arbacia eggs is the m gration of the red pigment granules to 

 the surface. In the mature unfeftilized and the immature eggs 

 they lie distributed throughout the cytoplasm. This change 

 takes place within ten minutes after fertilization and invariably 

 whenever membrane formation takes place, no matter by what 

 means brought about, whether by acid, by osmotic treatment or 

 by sperm. It is thus associated with membrane formation and 

 may be explained as follows: Most small particles suspended m 

 fluid media become negatively charged and there is additonal 

 evidence that these pigment containing bodies are so charged. 

 Lillie'^ has brought together evidence that the centrosomes are 



1^ R. Lillif: Am. yourn. Physiol., xv, p. 46, 1905. 



