FERTILIZATION 287 



of those reactions within the egg system upon which development 

 depends;" As a result of his direct analysis of the gametes durirg the 

 fertilization period Lillie has identified a substance in the egg which he 

 calls fertilizin. This substance is present in the egg for a short time 

 only; its formation usually begins at about the time the germinal vesicle 

 begins to break down, and immediately after fertilization its production 

 ceases, possibly through the neutralizing action of a second substance, 

 called "anti-fertilizin." As a rule it is only during the period at which 

 fertilizin is present that spermatozoa will enter the egg; the egg re- 

 mains fertilizable for but a short time. Hence it seems clear that it is 

 not the fertilization membrane that prevents the entrance of other 

 spermatozoa, as Fol thought, but rather the physiological state of the egg. 

 That the protection is thus a physiological rather than a mechanical 

 one is indicated by the fact that membraneless egg fragments without 

 fertilizin are not entered by spermatozoa. 



Fertilizin has two effects: it first acts by causing an agglutination 

 of the spermatozoa at the surface of the egg, and later causes the activa- 

 tion of the egg. It may thus be said to stand between the spermatozoon 

 and the activation reactions in the egg. Being present in the egg secre- 

 tion at a certain period it binds the spermatozoon to the surface of the egg, 

 and the spermatozoon, without necessarily penetrating the egg at all, 

 by means of a substance which it bears releases the activity of the 

 fertilizin within the egg, which results in development. In brief, the 

 activating substance is already present in the egg and is not brought 

 to it by the spermatozoon. It may be incited to activity by the sperm- 

 atozoon, but by other agencies as well. 



In concluding this sketch of the physiological features of fertiliza- 

 tion we may state briefly the immediate physiological consequences of the 

 process as summarized by Lillie (1919, Chapter V). The rate of oxida- 

 tion increases in most cases in which it has been investigated. In the sea 

 urchin egg (Warburg 1908-1914) this rate increases as much as six- or 

 seven-fold ; in Strongylocentrotus, four- or five-fold (Loeb and Wasteneys, 

 1912, 1913); in the starfish, apparently not at all. The egg membrane 

 becomes more permeable to oxygen, CO 2 , pigment, water, alkalis, intra- 

 vitam stains, and a number of other substances. The protoplasm be- 

 comes less fluid after fertilization (Heilbrunn 1915). This gelation effect 

 Chambers (1917) believes to center upon the sperm aster. The volume 

 of the egg decreases and its electrical conductivity rises. The most 

 conspicuous chemical change is seen in the loss of the fertilizin, and with 

 it the loss of capacity for further fertilization reaction. 



FERTILIZATION IN PLANTS 



Although the central act of the process of fertilization is regularly 

 the union of two sexually differentiated nuclei, the morphological 



