SUMMARY OF CURRENT RESEARCHES. 449 



they are segmenting. A membrane forms around the fertilized blasto- 

 mere, and the segmenting eggs perish rapidly. The changes underlying 

 development do not in themselves form a block. The eggs of S. pur- 

 puratus in which artificial membrane formation has been induced by 

 butyric acid, can also be fertilized subsequently with sperm. In the 

 case of Arbacia, membrane formation maybe induced in unfertilized ova 

 by treatment with butyric acid, and such ova disintegrate rapidly. But 

 the subsequent fertilization of such eggs by sperm saves their lives and 

 makes them develop. 



The egg of S. purpuratus can be fertilized by the sperm of Asterias 

 while both eggs and sperm are in a hyperalkaline solution. When the 

 eggs are put from the hyperalkaline sea -water, free from sperm, into 

 the normal sea-water with very motile sperm of Asterias, there is no 

 fertilization. When put back into hyperalkaline sea-water containing 

 Asterias sperm they will be fertilized rapidly. In other words, the effect 

 of the alkali is rapidly reversible. It is confined to the surface of the 

 cells. The block caused by the entrance of a sperm is of a similar 

 nature. — some rapid alteration of a physical property of the surface. 



Loeb goes on to inquire whether the motility of the spermatozoon 

 plays no other role than to bring the spermatozoon so close to the 

 surface of the egg that surface-tension phenomena can engulf the 

 spermatozoon into the egg. The spermatozoon cannot enter the egg 

 unless certain physical conditions at the boundaries of egg, sperma- 

 tozoon, and surrounding solution are right. The impact of the sper- 

 matozoon against the egg is a prerequisite for the process of fertilization. 

 A sea-urchin spermatozoon becomes more active when it comes near an 

 egg of its own species, and the immature eggs of Asterina activate the 

 sperm of the sea-urchin S. franciscanus as powerfully as is done by 

 the mature eggs of the sea-urchins S. purpuratus and 6'. franciscanus, 

 which shows that there is no strict specificity. Yet the spermatozoa of 

 Asterias ochracea and Asterina are activated strongly by the (immature) 

 eggs of their own species, and only slightly by the eggs of S. purpuratus. 

 So there is some degree of specificity. It must be remembered that all 

 these experiments are made in a NaCl solution, and that it requires a 

 stronger influence to activate the spermatozoa of a starfish than those of 

 a sea-urchin. Loeb believes that the activating effect of the egg upon 

 the spermatozoon is of the greatest importance for fertilization in 

 nature, and that the degree of specificity which exists (although it is 

 far from absolute) is a means of preventing hybridization. It seems 

 probable that the eggs, which are naturally fertilized in water, are 

 fertilized almost instantly after they are shed. There appears to be 

 simultaneously shedding of the germ-cells in the two sexes of Echi- 

 noderms. The specificity of the egg activates the sperm of the same 

 species much more quickly than that of a foreign species, and hybridiza- 

 tion is averted. 



The vibrations of the spermatozoon's flagellum assist boring into the 

 egg ; they may be needed also to make the spermatozoon stick to the 

 surface of the egg until other forces come into play. No proof for 

 the existence of a positive chemotropism of the sea-urchin sperm for the 

 eggs of the same species has yet been given. There is sometimes 



