STUDIES OF FERTILIZATION 531 



from the relative proportions of ripe eggs. Presumably, the 

 ovarian tissue was relatively more bulky in this case. 



The ovocytes are surrounded with the same layer of jelly in 

 Arbacia as the mature eggs; hence we must conclude that the 

 jelly is not the source of the agglutinating substance to whatever 

 extent it may become charged with it after maturation. An im- 

 portant point is involved in this conclusion discussed farther on. 



I think we are justified in concluding that ovocytes do not 

 secrete the agglutinating substance. The conditions in Nereis 

 fortify this conclusion, inasmuch as the ovocytes with intact 

 germinal vesicle do not secrete the fertilizin, but as soon as in- 

 semination take place it is poured out abundantly and the germi- 

 nal vesicle breaks down. 



In this connection it is important to note that in some animals 

 ovocytes with intact germinal vesicle do not respond to the 

 spermatozoon, even if cut in two so that the protoplasm is laid 

 bare. Wilson ('03) has shown this for the nemertean, Cerebra- 

 tulus, and Delage ('01) for the eggs of Strongylocentrotus and 

 Asterias. As I show farther on that the fertilizin is necessary 

 for fertilization, we may find in these facts evidence that the fertil- 

 izin is not only not secreted but is not even preformed in ovocytes 

 with intact germinal vesicle, at any rate not in the forms mentioned. 

 Its absence would furnish sufficient explanation of the failure of 

 such egg-fragments to fertilize, and the consequent impossibility 

 of producing merogony until the germinal vesicle breaks down. 



Delage ('01) also shows that, as soon as the germinal vesicle 

 begins to fade, in Asterias, and the nuclear sap diffuses into the 

 cytoplasm, the egg becomes fertilizable and merogony can be 

 induced. According to my interpretation the formation of fer- 

 tilizin must begin at this time. At any rate the non-fertilizable 

 condition of the cytoplasm before the germinal vesicle begins 

 to fade is associated with the absence of fertilizin. 



It is not necessary to assume that the fertilizin is contained in 

 the nuclear sap; on the contrary, the long-continued production 

 of this substance, as described in the next section, runs counter 

 to this idea. It is more probable that it exists in a state com- 

 parable to zymogen ready to be converted into active form, and 



