BLASTOrORE AND ANUS IN PALUUINA VIVIPARA. 377 



On the Coincidence of the Blastopore and Anus in Palu- 

 dina vivipara. By E. Ray Lankester, M.A., F.R.S. 



(With Plate XXV.) 



Dr. N. Bobretzky, of Kiew^ to whom we owe invaluable 

 works on the developmental histories of Astacus, Palsemon, 

 and Oniscus, has recently published in the ' Archiv fur 

 Mikrosk. Anatomie' (vol. xiii, p. 95) a memoir entitled 

 ' Studien liber die embryonale Entwickelung der Gastro- 

 poden." His observations relate almost exclusively to the 

 genera Nassa,Natica and Fusus and were made in Dr. Dohrn's 

 Zoological Station at Naples. Six plates illustrate this 

 work, which is remarkable for the fact that the studies of 

 which it gives an account are the first in which the method 

 of cutting sections has been applied to the examination of 

 the very delicate and small eggs and embryos of Gastropod 

 Molluscs. Dr. Bobretzky has also used and recommends a 

 method which consists in hardening the embryo in dilute 

 chromic acid and then observing it under the microscope as 

 an opaque object. 



M either of these methods could be applied to all molluscan 

 ova, for some are even smaller than those Dr. Bobretzky 

 has had patience and skill enough to cut, whilst others are 

 enclosed singly in large quantities of albuminous material 

 from which they could not be removed in quantity either 

 before or after the action of chromic acid. It is, therefore, 

 fortunate that three genera have come to hand at Naples 

 which could be treated in this way, and they have yielded in 

 Dr. Bobretzky's hands results of the very greatest interest. 



At the same time, whilst we feel bound to accept pro tanto 

 the main facts with regard to Nassa, Natica, and Fusus 

 which Dr. Bobretzky sets forth in drawings and letter-press, 

 we are not on that account, disposed to follow him in gene- 

 ralizing his results so as to include in their dominion all the 

 Gastropoda and even less all the MoUusca. In the study of 

 embryology the practical lesson is daily being more and more 

 forcibly impressed upon naturalists — that the assertion of 

 generality throughout a class or phylum of organisms for a 

 phenomenon observed only in two or three or even more 

 members of that class or phylum is an exceedingly risky 

 proceeding. Embryology in its present phase of development 

 is full of surprises, and generalizations must be understood 

 as possessing only the value — a very real and legitimate 

 value — of hypotheses, which need again and again to be 

 tested by further observation. 



