THE GENERATIVE OEGANS OF THE OYSTER. 271 



millimetres in thickness^ in which very little connective tissue can 

 be distinguished. 



The genital ducts are lined by a cubical ciliated epithelium^ which 

 is continued into the main branches given off from them, and it may 

 be asserted that the cell-walls of all the ultimate ramifications of 

 the organ are derived from an epithelium continuous with and 

 derived from that lining the ducts. 



The genital products are developed from the wall of the follicles, 

 probably at the expense of the epithelial cells which line them. 

 Both ova and spermatozoa are developed in the same follicle. 



In the youngest stage observed the ovum of the oyster is a little 

 cell, 20 — 24 ju in diameter, flattened on the side of the canal-wall, 

 and rounded on its free surface. The protoplasm of the cell-body 

 is feebly granular, the nucleus is large, spherical, and has a highly 

 refringent single nucleolus of a moderate size. The youngest ovules 

 pass by in sensible gradations into the more advanced, and these 

 again into the mature ova. The granulations of the protoplasm 

 become more and more numerous and more distinct, and little refrac- 

 tive granules accumulate in great numbers. The ripe ovarian ovum 

 acquires, as soon as circumstances are favorable, a spherical form, 

 and is rather more than O'l mm. in diameter. The manner in 

 which the ova separate from the follicles has not been observed. 

 After their separation there remains a lining of epithelial cells in 

 the follicle, from which new generative products are formed. 



It appears to be invariably the case, as will be shown afterwards, 

 that the formation of ova is always followed by the formation of 

 spermatozoa, but the process of spermatogenesis is much more 

 complicated than that of oogenesis. ' The most favorable prepara- 

 tions for the study of spermatogenesis are procured from follicles 

 which have been previously filled with ova. To procure this one 

 must search in the manner common among ostreiculturists for an 

 oyster with spat in its gills ; when found it must not be opened at 

 once, but must be marked and placed in an aquarium in a current of 

 water. If it is examined a fortnight later the different follicles of 

 the reproductive organ are found to be actively engaged in spermato- 

 genesis. A follicle in this condition is found in PI. XXIII, fig. 3. 

 In the centre is a loose mass of ripe spermatozoa, and the tissue 

 composing the walls is seen to be in the process of transformation into 

 spermatozoa. The minute mother-cells of the spermatozoa have a 

 diameter of scarcely 8 /n, they stain deeply with alum carmine, and 

 have dark granular contents and a small nucleus. The spermatozoa 

 are developed from them as follows. In each cell after the division 

 of the nucleus the cell-body divides in two portions. Of these one 

 is destined to give rise to numerous spermatozoa, the other seems to 



