900 REPORT OF COMMISSIONER OF FISH AND FISHERIES. [10] 



Dutliiers says tbat bis oyster larvae lived for more than a month in 

 aquariums, but he could not observe any important changes in their 

 organization, which certainly does not indicate a normal condition. I 

 once succeeded in observing' the development of oyster larvae for a 

 couple of hours, by making a small opening in the edge of tbe shell, 

 which hurt the animal scarcely at all. I was thus enabled to introduce 

 a small pipette into the mantle cavity and remove the larvae, but this 

 lasted only a short time, because with each operation so many embryos 

 were removed that all were soon taken. It is, therefore, not possible to 

 Ijrocure a continuous series of the different phases of the development. 

 In addition there is another difficulty, namely, that one cannot always 

 recognize the mother oysters from the outside. The relaxation of the 

 adductor, and consequently the less firm closing of the valves, is a pretty 

 sure indication that the oyster contains embryos ; but this phenomenon 

 is more marked in those oysters which contain more highly developed 

 embryos, which are on the point of being ejected from the shell. For 

 this reason I have often perceived more mother oysters containing ad- 

 vanced embryos than those which contained young embryos. And for 

 this reason likewise the first stages of the segmentation of the egg have 

 to a great extent remained unknown to me. 



C. Davaine-" and Professor Mobius^^ have, however, given illustrations 

 and descriptions of some of the early phases of the segmentation of the 

 egg of the oyster. It is there seen that after the polar globules have 

 been ejected (Fig. 2), the egg divides, like those of other lamellibran- 

 chiata,^^ into two spheres; the smaller, the animal, and the larger, the 

 vegetative si)here (Fig. 3). This stage is followed by another, during 

 which the egg is composed of four spherules, three smaller than the an- 

 itnal pole, resting on a large vegetative spherule (Fig. 4). By repeated 

 segmentation of theanimal spheres, and theformatiou of smaller spheres, 

 originating by budding from the vegetative sphere, there is formed on 

 the animal pole a layer of small cells which gradually envelop more and 

 more the vegetative sphere, without, however, inclosing it entirely (Fig. 

 5). Then the vegetative sphere also begins to divide, first into two large 

 segments (Fig. 6), later into several cylindrical cells. Thus two layers 

 are formed, one of which will develop into the ectoderm and the other 

 into the entoderm (aud into the mesoderm ?). At the same time thf* 

 embryo loses its spherical form and, owing to a depression on the lower 

 side, becomes slightly reniform (Fig. 7, where, however, the lower part 

 is turned upside down). If one observes an optic section of a more ad- 

 vanced stage (Fig. S), one sees that the layer of entodermic cells has a 

 slight depression and that a genuine gastrula has been formed. There 

 can, however, be no question of a genuine invagination, because there 



-" See his work already leferred. to, p. 34, Plate II. 

 21 See his work already referred to, -p. 16. 



2^ The Pisidiuvi seems to form au exceptioii. Ray Laukester : Contributions to the 

 develop, history of the Mollusca, in Phil. Transact., vol. 165, 1876. 



