1 64 DYNAMICS OF LIVING MATTER 



the role of surface tension for the entrance of the spermatozoon is cor- 

 rect, these restrictions to the fertilizing power of the spermatozoon are 

 only apparent, and that we have only to find modifications of the natu- 

 ral media, which allow the spermatozoon to enter the eggs of foreign 

 species. 



As a rule only one spermatozoon enters an egg: as soon as this has 

 entered no further spermatozoon can enter. This is also true for frag- 

 ments of an egg. First, O. and R. Hertwig and later Boveri, Delage, 

 and many other authors showed that a piece of an unfertilized egg can 

 be fertilized by a spermatozoon. . Janssens has recently observed that if 

 a piece of protoplasm be cut off from a fertilized egg, this can no longer 

 be fertilized. It is not impossible that the entrance of a spermatozoon 

 alters the surface tension of the protoplasm of the egg, making it thus 

 impossible for another spermatozoon to enter. 



The egg of a starfish is, as a rule, not yet ripe, i.e. capable of being 

 entered by a spermatozoon immediately after it is taken from the ovary. 

 It has to lie for about two hours in sea water before it is ready for fertili- 

 zation. During this time the polar bodies are thrown out. Delage has 

 shown that if a piece of protoplasm be cut off from an egg of a starfish 

 (Asterias glacial-is] before it is ripe, it cannot be fertilized by a sperma- 

 tozoon, but that this can be done when the piece of protoplasm is cut off 

 from the egg after the egg has gone through the process of maturation.* 



It is generally stated that the pollen of a hermaphroditic plant can- 

 not fertilize the egg cells of the same individual. Castle found that 

 similar though less pronounced conditions exist in a hermaphroditic 

 Ascidian ; namely, Ciona intestinalis. The eggs of a Ciona can, as a rule, 

 not be fertilized with the sperm of the same individual, while they can 

 be fertilized with the sperm of another individual. This immunity of 

 the eggs against sperm of the same individual is not without exception. 

 In some cases Castle found that 5, 10, or even 50 per cent of the eggs 

 of an individual could be fertilized with sperm of the same individual. 

 Morgan confirmed Castle's observations, and found that if the eggs are 

 put for about ten minutes in a 2 per cent ether solution in sea water, in 

 a number of (but not in all) cases the number of fertilized eggs shows a 

 sliht increase. 





2. ARTIFICIAL PARTHENOGENESIS AND THE THEORY OF FERTILIZATION 



It is hardly necessary to state that at all times authors have been 

 ready to explain the fertilizing or developmental action of the sper- 



* Delage, Archiv. de Zool. experimental, Vol. 7, pp. 383, 511. 

 t Morgan, Jour, of Exper. Zool., Vol. I, p. 135, 1904. 



