MEMOIRS. 



On the Development of the European Oyster {Ostrea 

 edulisy L.) By Dr. R. Horst, of the Utrecht Museum 

 of Zoology. (With Plate XXVII.) 



Last summer, when the Dutch Zoological Station was 

 erected in the neighbourhood of the East Scheldt oyster- 

 banks, I studied for some weeks the development of the 

 oyster. Although this research is not fully completed, I 

 believe that the following communication may contribute to 

 augment our still very scanty knowledge of the embryology 

 of Lamellibranchiata. 



In studying the development of the oyster we meet with 

 peculiar difficulties, which made the French zoologist — 

 Lacaze-Duthiers — say justly, " L'huitre est certainement 

 I'une des especes du groupe des Acephales lamellibranches la 

 plus difficile a etudier dans son organisation comme dans son 

 developpement."^ The embryos not only pass their first 

 stages of development within the pallial cavity of the mother, 

 but the impregnation of the eggs takes place also internally 

 instead of externally ; perhaps the eggs and the spermatozoa 

 even meet in the efferent ducts of the genital gland. To 

 observe the first changes of the impregnated egg one 

 cannot make use of artificial fertilisation, but one is obliged 

 to open by chance several breeding oysters. Having opened 

 a mother-oyster in the usual way, that is, by means of cutting 

 the adductor muscle, it soon dies, and so the normal develop- 

 ment of the spat which it contained is accordingly disturbed. 

 One can keep the embryos alive for some days outside 

 the mother in an aquarium, but not without their showing 

 very soon either pathological states or the development 



1 " Memoire sur le Developpement des Acephales lamellibranches," 

 'Compt. Rend, Acad. So.,' Paris, t. xxxix, p. 1197. 



