THE FERTILIZATION-REACTION 



Fertilizability being resident in the cytoplasm, we may 

 ask whether or not it has definite location. As we have 

 seen, in the egg of Platynereis, for example, loss of fertiliz- 

 ability may occur very quickly. Since an egg is either 

 fertilizable or not, it is easy to assume that for eggs generally 

 the loss, like the onset, of fertilizability is the event of a 

 moment. Now the rapidity of the loss strongly indicates 

 the superficial location of that upon which fertilizability 

 depends. 



Two cases show that with the onset of fertilizability the 

 ectoplasm of the egg exhibits visible alteration in structure. 

 At Naples I often noted that eggs of sea-urchins after com- 

 plete maturation but before they are fertilizable show very 

 minute projections from their surfaces. Fertilizable eggs, 

 in contrast, present a smooth contour; the ectoplasm is 

 homogeneous in appearance. The egg of Chaetopterus 

 undergoes a series of remarkable changes, its ectoplasm 

 flowing from the animal pole to cover the hitherto exposed 

 vegetal pole, before it becomes fertilizable by spermatozoa 

 entering at the vegetal pole. With the flowing movements 

 in the ectoplasm, fertilization may take place at other 

 points. 



No one has to my knowledge made a systematic study of 

 a great number of species of eggs with respect to the occur- 

 rence of structural changes in the ectoplasm which might 

 be correlated with fertilizability. Until this has been done, 

 we can not reject the possiblity that such changes generally 

 intervene. Also, these changes may be of such extreme 

 delicacy that they are overlooked. Finally, the change to 

 the fertilizable condition may not always express itself as 

 visibly structural. 



The rapidity with which fertilizability arises and disap- 

 pears points to the conclusion that it is a condition of the 

 ectoplasm. Then, also, we may assume that fertilization 

 is concerned with the ectoplasm. But we need not rest 



191 



