ZOOLOGY: E. G. CONKLIN 
493 
Franco tte (1898) discovered in the poly clad Prostheceraeus that at 
the first maturation the egg sometimes divided into two nearly equal 
cells; each was then entered by a spermatozoon and normally fertilized 
and at the second maturation division each formed a small second 
polar body and the larger cells then underwent normal cleavage and 
developed to the gastrula stage. In a few other instances the entrance 
of a spermatozoon into a polar body has been reported though some of 
the cases are not entirely convincing and need verification. Thus Platner 
(1886) described the entrance of a spermatozoon into a polar body of 
Arion; he maintained that the polar bodies are formed before the en- 
trance of the sperm, which would make this case similar to that of 
Prostheceraeus, but the evidence is by no means conclusive. Sobotta 
(1895) calls special attention to the large size of the polar bodies in the 
mouse and suggests that they may be capable of being fertilized but 
offers no evidence in favor of this view. Kostanecki (1897) has ob- 
served a sperm^atozoon with its head penetrating the second polar body 
of Physa, a thing which he regards as merely a 'curiosity.' 
Lefevre (1907) observed that the polar bodies of Thalassema undergo 
several cleavages resulting in the formation of a morula-Hke cluster of 
minute cells when they are exposed to weak solutions of HCl/' thus 
they respond to the same divisional stimulus suppKed by the acid solu- 
tions as does the egg cell itself." 
The most striking difference between Prostheceraeus and other 
animals in which giant polar bodies have been reported is to be found 
in the fact that in the former fertilization does not take place until after 
the first maturation division is completed and then each of the daughter 
cells is fertilized, whereas in the latter the entrance of the spermato- 
zoon occurs before the completion of the first maturation division with 
the result that one of the daughter cells contains a spermatozoon and 
the other does not. 
In this fact is to be found the explanation of the different behavior 
of the giant polar bodies of Prostheceraeus and of Crepidula, for it is 
well known that one of the first effects of the entrance of a spermato- 
zoon into an egg is the prevention of other spermatozoa from entering. 
If the spermatozoon enters the egg before the first polar body is cut 
off that polar body as well as other cells which are formed later from the 
egg are rendered ''immune" to other spermatozoa. 
But although the influence of the entering spermatozoon spreads so 
rapidly over the egg that within a few minutes at most it renders all 
portions of the egg surface 'immune' to other spermatozoa and thus 
prevents the fertilization of polar bodies which are formed after fertili- 
