XXXIV.-THE DEVELOPMENT OF THE OYSTER (OSTREA EDU- 



LIS L).* 



By Dr. R. Horst. 



I. — Historical. 



More has been written on the history of the development of the oyster 

 than on that of any other invertebrate. The cause of this doubtless is 

 the circumstance that from time immemorial the oyster has been con- 

 sidered a great delicacy, and that therefore attention was early directed 

 to its develoi3ment. 



Brach ^ (1690) seems to have been the first to use the microscope in 

 observing the embryo of the oyster. According to this author, the oys- 

 ters found in the Venetian waters produce spawn towards the end of 

 spriug, during the entire summer, and in the beginning of autumn, 

 which spawn they discbarge into the water. During this time the central 

 round and fleshy part of the oyster diminishes in volume and loses its 

 succulence, while the brauchia and the edge of the mantle assume a 

 harder and more solid appearance. The liquid seen in the shell round 

 the body in the beginning looks whitish, clear, and fluid, but later 

 changes to a blackish mass having the consistency of pap. 



When this mass is examined under the microscope, one finds in it 

 eggs in two diflereut stages of development : (1) Some entirely white, not 

 altogether spherical, and resembling pellets; these eggs are quiescent 

 and consequently less developed ; (2) Others which are also white and 

 round, but a little more compressed; they have the form of a grown 

 oyster and show a black line in the place where the little valves will 

 open. These last-mentioned eggs, moreover, move about and maintain 

 a spiral motion in the mother shell. This continues until the eggs are 

 more developed {usque ad perfectiorem animationis gradum). Later the 

 oyster closes its shell, so that the water from the outside cannot pene- 

 trate into it, and the mother oyster becomes very lean, because she has 

 to feed her offspring. During this time the young oysters gradually 



" ^' De OntwikJcelingsgeschiedems ran de Oesfer." From Tijdsdhrift der Nederlandsche 

 Dierkundige Vereeniging. Supplement No. 1, Leyden, 1883-84. Translated from the 

 Dutch by Herman Jacobsox. 



' D. Jac. Brachii: Obstrv. CCIII, De oris Osfreorum. Ephemer. Acad. Leop. Nat. Cur., 

 Ann. VIII, 1690, p. 506. 



[1] 891 



