THE FERTILIZATION-PROCESS 



eggs according to the stage in maturation in which they 

 are fertilizable. 



Class I : The &%g reaches the stage just prior to the matu- 

 ration-divisions. This is the so-called germinal vesicle 

 stage characterized by a large nucleus. It there comes to 

 rest and dies unless fertilized. Mere contact of the sper- 

 matozoon is sufficient to cause the break-down of the 

 germinal vesicle and the initiation of the maturation divi- 

 sions. Eggs of various worms, Polys to mum, Gyrodactylus, 

 Ascaris, Sagitta, Nereis, Platynereis, Thalassema, Myzo- 

 stoma, and of the clam, Mactra, are examples of this class. 



Class 2: Here belong eggs which will not fertilize as long 

 as the germinal vesicle is intact. They develop as far as 

 the stage of first maturation and there remain until death 

 unless fertilized. The eggs of the sea-worms, Thysanosoon, 

 Prostheceraeus, Chaetopterus, Phascolosoma, and of the clams, 

 Cumingia and Mytilus as well as eggs of snails and of the 

 ascidians, Ciena and Phallusia, are examples of this class. 



Class 3 : Some eggs finish the first maturation before they 

 reach the stage in which they are capable of fertilization. 

 They come to rest with the second maturation spindle 

 formed, one polar body having been extruded. Here 

 belong eggs of many vertebrates; likewise the egg of the 

 worm-like chordate, Amphioxns, regarded by some zoolo- 

 gists as the ancestral form of the vertebrates. 



Class 4: Eggs of sea-urchins constitute an example of 

 this class in which fertilization is possible only after comple- 

 tion of both maturation divisions. These are called fully 

 matured eggs. 



It is here necessary to make a sharp distinction between 

 maturation, referring to the phenomena of polar body 

 formation, and physiological ripeness, referring to the 

 capacity of the egg-cytoplasm for fertilization, which as 

 this classification shows reaches its optimum at various 

 stages in the maturation-process (Fig. 21). 



153 



