ON THE DEVELOPMENT OF THE POND-SNAIL. 365 



Observations on the Development of the Pond-snail 

 {LymrKBUs stagnalis), and on the Early Stages of other 

 MoLLUscA. By E. Ray Lankester, M.A., Fellow and 

 Lecturer of Exeter College, Oxford. (With Plates XVI 

 and XVII.) 



§ 1. — Some of the Developmental Phenomena of 



MoLLUSCA. 



Four years since, I determined to make a study of the de- 

 velopmental phenomena of a series of Mollusca, with the 

 view of ascertaining from the minute comparison of a number 

 of cases what phenomena might be common to the group, or 

 be considered as indicating ancestral conditions inherited from 

 common ancestors. 



The success which had attended Fritz Miiller's investiga- 

 tion of the Crustacea, and his celebrated " recapitulation 

 hypothesis," according to which we have, in the development 

 of every individual organism, a more or less complete 

 epitome of the development of the species, so that the series 

 of changing forms passed through between ovum and adult 

 form are but a series of dissolving views or portraits (often 

 very much marred) of its line of ancestors — this, I say, led 

 me to hope that materials might be found in the develop- 

 mental history of the Mollusca for constructing their genea- 

 logical tree. During the past fifteen years but little has 

 been done in the study of the embryology of the Mollusca, 

 and it was therefore to be expected that the application of 

 improved methods of investigation and new hypotheses would 

 yield valuable results. The result of my study of the de- 

 velopment of the Lamellibranch Pisidium and of the 

 Gasteropods Aplysia, Neritina, Tergipes and Polycera, are 

 now in course of publication elsewhere. 



I have also, during this spring, completed the examina- 

 tion of the development of the Cephalopod Loligo from an 

 early stage of the ovarian egg up to the escape of the embryo 

 from the egg-jelly, which, together with less complete ac- 

 counts of the development of Octopus and Sepia, I hope soon 

 to see published. Before proceeding to give here an account 

 of observations on LymticBiis which I carried out during 

 July in the laboratory of Exeter College, Oxford, I may 

 briefly summarise the chief results of my previous observa- 

 tions, which are remarkably confirmed by the facts to be 

 subsequently related in regard to Lymnceus. 



Kowalevsky, in his account of the development of Amphi- 



