THE FERTILIZATION PROCESS IN THE SNAIL. 69 



ginning with the efforts of Laurent ('37) and others, and extend- 

 ing to the last few years, and a considerable amount of excellent 

 work has been done on fertilization in normal or so-called cross- 

 fertilized species, very little work has been done on reproduction 

 in isolated individuals. Lillie and Just ('25, p. 218) state that 

 "... pulmonates appear to reproduce exclusively by cross-ferti- 

 lization. . . ." However, they point out that the work of Braun, 

 Colton and Cook in obtaining fertile eggs from isolated parents 

 indicates that self-fertilization does occur in this order of mollusks. 



The results obtained by various investigators indicate that the 

 solution of the problem of fertilization in pond snails will be 

 found by careful cytological studies on the eggs and oocytes of 

 isolated stock. 



I wish to take this opportunity to thank Professor A. Franklin 

 Shull for having directed this work. I am also indebted to the 

 Graduate School of the University of Michigan for help from its 

 research fund, and to staff members of the Departments of 

 Zoology and Botany for favors, to the Marine Biological Labora- 

 tory for library and other facilities, and to several investigators 

 at this laboratory and others in the Zoological Laboratory of the 

 University of Pennsylvania for suggestions drawn from their 

 various fields of specialization. 



I. MATERIAL AND METHODS. 



A. Material. 



Individuals of the parent stock from which the different species 

 used in these experiments were obtained were identified by Mr. 

 W. J. Clench and these identifications were later verified by 

 Doctor Bryant Walker. The nomenclature used in referring to 

 North American forms is that of Walker ('12). In every in- 

 stance " virgin " snails have been used, unless otherwise stated in 

 the explanations. This " virgin " material was obtained by iso- 

 lating an embryo and allowing it to hatch in a clear glass finger- 

 bowl in which it remained until used or set free. Thus the eggs 

 and ovotestis tissue from these " virgins " comprise the material 

 upon which the results of this investigation are based. 



The tissue and eggs of Lyuincca stagnalis appressa Say were 

 used for this article, but the living and prepared eggs as well as 

 6 



