150 The Mechanistic Conception of Life 



If salts are added to the sea-water or if it is diluted by 

 the addition of distilled water the tension and the diameter 

 of the membrane do not change. This proves that the 

 membrane is permeable for salts, but not for colloids, and that 

 the lifting up of the fertilization membrane is determined by 

 the swelling and subsequent liquefaction of a colloid. This 

 dissolved colloid exercises an osmotic or colloidal pressure and 

 sea-water must diffuse from the outside imder the fertilization 

 membrane of the egg until the tension of this membrane equals 

 the osmotic or colloidal pressure of the dissolved colloid. This 

 explains also why it is that the fertilization membrane as a rule 

 assumes a spherical shape. 



We now can understand why not in all cases of fertilization 

 a distinct fertilization membrane is formed. This may be due 

 to the fact that the degree of swelling of the colloid of the cortical 

 layer varies under different conditions. 



XI 



We now possess a pretty complete picture of what happens 

 to the egg in the case of ''formative stimulation," i.e., when it 

 is caused to develop. Through a lysin or some other cytolytic 

 agency a certain substance of the cortical layer, presumably a 

 lipoid, is dissolved or precipitated, whereby a protein substance 

 of that layer is able to absorb water and swell. Formerly it 

 was thought that the spermatozoon caused the development 

 of the egg by carrying a ferment or enzyme into it and that this 

 ferment set the mechanism of development into action. Others 

 expressed the opinion that the entrance of the sperm nucleus or 

 of a centrosome was responsible for the development. We see, 

 however, that it suffices to call forth the artificial membrane 

 formation in the unfertilized sea-urchin egg, in order to observe 

 after two or three hours the formation of normal astrospheres or 

 spindles. This disproves the suggestion that the fusion of egg 

 and sperm nucleus is essential for the development of the egg.^ 



1 The fusion of the nuclei is of course of importance for the transmission of 

 paternal qualities. 



