THE FERTILIZATION PROCESS IN THE SNAIL, 

 LYMNJEA STAGNALIS APPRESSA SAY. 1 



EDWARD DRANE CRABB, 

 DEPARTMENT OF ZOOLOGY, UNIVERSITY OF PENNSYLVANIA. 



CONTENTS. 



Introduction ^ 67 



I. Material and methods 69 



A. Material 69 



B. Methods 70 



a. Fixation 70 



b. Imbedding 70 



c. Staining 71 



II. Copulation 71 



III. Oviposition and Viability of Eggs 72 



IV. Self-fertilization and Parthenogenesis 74 



V. Protandry 74 



VI. Development and Migration of the Germ Cells 75 



VII. Maturation of Virgin Eggs 78 



VIII. Insemination 8l 



A. Conoid and Round-Head Spermatozoa 81 



B. Ovarian Insemination 82 



C. Normal Insemination 83 



D. Changes in the Oocyte following Insemination 84 



E. Formation of the Sperm Amphiaster 84 



F. Formation and Fusion of the Pronuclei 86 



IX. The Chromosomes 88 



In Egg Cells 88 



a. First Polocyte 88 



b. Second Polocyte 89 



r. Karyomeres 89 



X. Conclusions and Summary 91 



XL Literature Cited 92 



XII. Plates I to 6 with Explanations 98 



INTRODUCTION. 



The primary object of this study is to determine whether her- 

 maphroditic pond snails which have been reared and kept in strict 

 isolation reproduce by self-fertilization or by parthenogenesis. 

 On this point previous investigators have reached different con- 



1 Contribution from the Zoological Laboratory of the University of 

 Michigan. 



67 



