THE FERTILIZATION-PROCESS 



the membrane separates with extreme rapidity. The lat- 

 ter also says that separation begins at the point of sperm- 

 entry, whilst according to Cerfontaine, actual lifting begins 

 at the animal pole.^ Appended figures (Fig. 28) of surface- 

 changes are of the egg of Petromyzon, a member of this 

 class. 



After sperm-entry the bulk of the spermatozoon is car- 

 ried inward leaving a remnant at the entrance-point which 



k I 



Fig. 28. — Surface changes, egg oi Petromyzon, during stages of sperm-penetration 



(after Calberla). 



is still visible as late as the four-cell stage. Two equal 

 asters arise from the division of the single aster found in 

 the vicinity of the spermatozoon. Whether or not this 

 aster contains a centrosome is not certain. From an excel- 



^ Hatschek has also recorded some interesting observations con- 

 cerning elasticity of the membrane. 



i6g 



