THE MORPHOLOGY OF FERTILIZATION 3 



Hamilton & Day, 1945). In the eggs of Ciona, Chaetopterus, 

 Cumingia and Mytilus, fertilization occurs at the first maturation 

 metaphase (Class 2); while in sponges, Nereis, Spisula, Urechis 

 caiipo Fisher & MacGinitie (Plate II), Ascaris and Sagitta, the egg 



Class 3 



Class 2 



Class / 



Oocyte Ut. Maturation 2nd.Maturation 



metaphase metaphase 



Ootid 



Fusion of 

 pronuclei 



FIG. I. — The four stages of egg maturation at which fertilization occurs in the 

 animal kingdom, after Dalcq (1952). 



is fertilized before the germinal vesicle of the oocyte has broken 

 down, that is before either maturation division (Class i).* These 

 four classes should be remembered when studying fertilization, 

 as we are sometimes tempted to make generalizations based on 



* Needless to say, there are exceptions to this classification, e.g. starfish eggs, 

 which come into Classes i, 2, 3 and 4. 



