338 The Molluscan Family Planorbidae 



PLATE 51 



Development of Helisoma scalare (Jay) 



1. Egg capsule containing 28 eggs, observed on March 1, 1932. Each egg is marked 



to correspond with the numbered figures below. Line near figure indicates 1 mm. 

 in length. 



2. The 24-cell stage of segmentation in egg No. 7. March 7. 



3. Gastrula stage of segmentation in egg No. 7. March 8. 



4. Post-trochophore stage in egg No. 11. March 4. 



5. Post-trochophore stage in egg No. 11. March 5. 



6. Veliger stage of embryo in egg No. 11. March 9. 



7. Veliger stage of embryo in egg No. 8. Front view. March 9. 



8. Veliger stage of embryo in egg No. 8. Side view. March 9. 



9. Veliger stage of embryo in egg No. 8. Top view. March 10. 



10. Embryo in egg No. 3. March 12. 



11. Embryo in egg No. 1 showing heart and stomach. March 11. 



12. Young snail in egg No. 3. March 14. A similar young snail was observed in egg 



No. 20 on March 14. Shell growth distinct. 



13. Young snail just out of egg No. 1. March 15. 



14. Young snail two days out of egg No. 1. March 16. Shell now shows both growth 



lines and spiral striae. 



Line at lower left indicates 1 mm. in length 



