46 * PROBLEMS OF FERTILIZATION 



divisions, and either do not begin, or do ^ot complete, 

 their maturation divisions, as the case may be, unless 

 fertilized. Thus the maturation and the fertilization 

 of the ovum frequently overlap. 



The following cases may be recognized: (i) The 

 ovum loses its capacity for division at the end of its 

 period of growth. The large nucleus, known as the 

 germinal vesicle, undergoes none of the preparatory 

 stages of karyokinesis unless the egg be fertihzed; 

 this is the case, for instance, in certain annelids and 

 nematodes, of which the annelid Nereis may serve as 

 a type. (2) The ova of the annelid Chaetopterus, of 

 the nemertean Cerebratulus, and of the lamelhbranch 

 Cummingia pass through the prophases of the first 

 maturation division, but the karyokinetic process is 

 arrested in the mesophase of this division, and the 

 ovum will die in this condition unless fertilized. (3) In 

 the case of the ova of many vertebrates the first 

 polar body is formed, and the prophase of the second 

 maturation division begins, but the process then stops 

 unless the egg be fertilized. (4) In the echinids and 

 some other animals maturation is completed without 

 fertilization. 



In the first three cases the spermatozoon remains 

 more or less quiescent within the egg during the comple- 

 tion of the maturation divisions, and the internal events 

 of fertilization are resumed after the formation of the 

 second polar globule. These variations in the time at 

 which the egg reaches the period of inhibition or quies- 

 cence affect the morphological features of fertilization 

 in certain important respects; they must be borne in 

 mind also in the interpretation of experiments. 



