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The Croonian Lecture for 1826. By Sir Everard Home, Bart. V.P.R.Sl 

 Read November 16, 1826. [Phil. Trans. 1827, p. 39.] 



The author states the subject of this lecture to be, " an inquiry 

 into the mode by which the propagation of the species is carried on 

 in the common oyster, and the large fresh-water muscle." 



After noticing the agreement of these animals, in the circumstance 

 of their ova becoming the nuclei on which pearls are formed, he ob- 

 serves that they disagree in the process gone through before the 

 young is completely formed. The mode of propagation of the oyster 

 he regards as more simple than that of many plants ; and the pro- 

 cesses, as gone through in much shorter time. 



In describing the mode of breeding of the oyster, the author first 

 observes that we labour under considerable disadvantage in the in- 

 quiry in this country, from the prohibition of their sale during the 

 spawning season ; from which, however, be was relieved by a weekly 

 supply of oysters from a private bed. 



He first describes the two ovaria, which he states to lie immediately 

 within the membrane that lines the two shells, having the liver be- 

 tween them, whose structure resembles so closely that of the ova- 

 rium containing ova, as only to be distinguished by their colour and 

 situation. In March, the ova were large enough to be visible in the 

 field of a microscope, and were spherical ; as they enlarge, that mem- 

 brane to which they are attached becomes thickened. In June, 

 they have attained their full size, and are seen surrounded by a white 

 liquor, which the author regards as impregnating fluid. At the time 

 of their detachment a tube is seen, not to be detected before, origi- 

 nating by an opening between the two ovaria. This is the oviduct ; 

 and the embryo, when it enters it, has already acquired a shell. About 

 the end of June, the young begin to leave the ovaria, and at the end 

 of July none are found, either in them or in the oviduct. 



The author then observes, that the oyster seems to form an ex- 

 ception to the general rule observed by other fish, being out of 

 season, and having no flavour during the time that the ovaria are full 

 of ova. In fact, however, the breeding season of oysters, when the 

 ova are getting ready for impregnation, is March or April ; while in 

 June or July they may be said to have spawned, the embryo being 

 then received into the mouth of the oviduct. 



In the fresh- water muscle the ovaria resemble, in situation and 

 appearance, those of the oyster ; and the ovaria are the same size 

 before impregnation, which in them also happens in the ovarium. 

 They are completely formed about the 10th of August, and about the 

 20th are detected passing into the oviduct ; and about the 1 2th of 

 September have all arrived there. That they are previously impreg- 

 nated is evident from the ovum having been formed into a vesicle, 

 through which the embryo may be seen opening and shutting its 

 shells. The author here notices a curious phenomenon, many of 

 the young being seen in this state, turning round and round as on a 

 centre. Mr. Bauer ascertained that this motion is produced by a 



