INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. i)9 



instead of by sections, as hitherto, shows large, pear-shaped, nucleated 

 spermatoblasts, but also provided with cilia, to occur. The cilia 

 penetrate deeply into the cell-substance, and meet at a certain dark 

 spot; another cell, still containing the nucleus, will show the cylin- 

 drical body of the spermatozooid traversing the cell, with the cilia at 

 one of its ends. It may, perhaps, prove to be developed from the dark 

 spot just mentioned. It is determined satisfactorily that the sperma- 

 tozooid is formed within the cell, and does not result simply from its 

 elongation, for it may be found in a still more advanced state still 

 enveloped to a greater or less extent by a sheath containing proto- 

 plasm with or without a nucleus. 



In both kinds of sperm-cells three parts — tail, body, and head — 

 may be shown by reagents to be present, and quite distinct from each 

 other. Exposure in a damp chamber f(jr thirty-six hours has the effect 

 of killing and causing the dissolution of the large forms, while the small 

 ones remain recognizable in all their parts, and sometimes motile. 



C. Siebold has attempted to show the two forms to be mere stages 

 of one, but later regarded the vermiform one as a spermatophore. 

 Leydig gives a fact confirmatory of their distinctness, viz. that tho 

 two forms are to be found within the envelope of the ovum, KoUiker 

 argues, from others' observations, that the large forms are mother- 

 cells for the small ones. Baudelot considers the large forms — his 

 " ciliiferous tubes" — to be a atage of the filiform, which are the 

 fully developed si:>ermatozoa. 



Embryolog-y of the American Oyster.* — All the writers upon 

 the development of the oyster, from Home (1827) to Mobius 

 (1877), state that the eggs are fertilized inside the shell of the 

 parent, and that the young arc carried inside the mantle-cavity 

 until they are provided with shells of their own ; that they leave the 

 parent in a somewhat advanced state of development, and that their 

 free-swimming life is of short duration, and lasts only until they find 

 a suitable place to attach themselves. 



Mr. W. K. Brooks, of the John Hopkins University, U.S., says 

 that in 1879 he carefully examined the gills and mantles of more 

 than one thousand oysters (from one bed), but never found a single 

 fertilized egg or embryo inside the mantle-cavity of an adult, although 

 he found females with the ovaries full of ripe eggs, others half empty, 

 and some almost entirely so, with all the intermediate stages ; so that 

 he concludes that there is an important difference in the breeding 

 habits of American and European oysters, the eggs of the former 

 being fertilized outside the body of the parent. During the period 

 which the European oyster passes inside the mantle-cavity of the 

 parent, the young American oyster swims at large in the open ocean. 



The more imjjortant points in the development of the oyster, also 

 established by the author by means of tho artificial fertilization of 

 a large number of eggs taken from the ovaries, are : — 



1. The oyster is i)ractically unisexual, since at the breeding season 

 each individual contains either eggs or spermatozoa exclusively. 



♦ 'Am. Juurn. Sci. aiiil Arts,' xviii. (1S71») \^. 425. 



