POLYVITELLINY IN POND SNAILS. 



EDWARD D. AND RUBY M. CRABB.i 

 DEPARTMENT OF ZOOLOGY, UNIVERSITY OF PENNSYLVANIA. 



CONTENTS. 



Introduction 318 



Material and Methods 319 



Normal and Polyvitelline Eggs 319 



Morphology 319 



Maturation and Cleavage 320 



Viability 320 



Occurrence of Polyvitelliny 320 



In Eggs of Physa sayii 320 



In Eggs of Lymncea columella 321 



In Eggs of First and Second Generations 321 



In Eggs of Lymncea stagnalis appressa. Table 1 321 



In Eggs of Lymniza palustris. Table II 322 



In Large and in Small Egg Masses 322 



Discussion 322 



Summary 323 



References Cited 324 



Plate i and Explanation 326 



INTRODUCTION. 



The occurrence of more than one vitellus, or embryo, in a 

 single egg of some of the fresh-water pulmonates has never been 

 satisfactorily explained. Although various forms of abnormal de- 

 velopment which might be considered to have some relation to 

 twinning, such as a Campeloma having two separate dextral shells 

 (MacCurdy, '09), have been reported, such abnormalities were 

 not observed in our laboratory stock or among the many wild 

 individuals examined. The fact that a wild snail, number 5, 

 Table I., was found to lay eggs having more than one vitellus 

 when brought into the laboratory, suggested the idea that the lay- 

 ing of such abnormal eggs might be a transmissible character. 

 When, however, in less than two months several of these abnormal, 

 or polyvitelline, eggs had been laid by the controls and by other 

 wild and laboratory reared snails, the genetic explanation seemed 

 less likely. Fortunately, some of the stock from the projected 



1 Contribution from the Zoological Laboratory of the University of Pennsylvania 

 and the Zoological Laboratory of the University of Michigan. 



318 



