[7] DEVELOPMENT OF THE OYSTER. 897 



is unisexual ; its eggs are fecundated aud develop outside the mother 

 oyster. The progress of segmentatiou depends a good deal on tbe tem- 

 perature aud other circumstances. In some cases the embryo, provided 

 with vibratile cilia, is formed two hours after fecundation ; but the 

 ordinary duration of the process is not less than twenty-four hours, aud 

 may even extend over more than two days. At the beginning of seg- 

 mentation the egg assumes an oval shape, the thick end becomes the 

 animal pole (formative, Brooks), aud the small end the vegetative pole 

 (nutritive, Brooks). This latter corresponds to the dorsal side of the 

 embryo. During violent movements of the vitellus the first polar glob- 

 ule issues from it. There does not seem to be any segmentation cavity 

 such as is observed in the eggs of other moUusks. After repeated seg- 

 mentations of the ammal spheres, whose number is still farther in- 

 creased by other spheres, originating in the vegetative part, a layer of 

 ectoderm cells is formed which covers the vegetative sphere, and as it 

 grows larger completely envelops it. The vegetative sphere is first 

 divided into two entodermic cells, whose number afterwards increases 

 to six and more. The embryo is flattened out, and the ectoderm and 

 entoderm become detached from each other. Finally, the entoderm is 

 invaginated, so that in form it resembles a shallow cup with a shallow 

 gastric cavity. The embryo now undergoes an important change ; a 

 crown of vibratile cilia Is formed, and in the dorsal part, transversely to 

 the great axis of the bodj', a depression begins to appear which repre- 

 sents the blastopore. This finally closes, and the entoderm, a mass of 

 cells, is inclosed in the j)eritoneal cavity. But prior to this there aj)- 

 pears, at each extremity of the depression of the blastopore, a small 

 irregular transparent body ; these are the two valves. A couple of 

 houis later, the embryo has considerably increased in size, and there 

 may be seen forming in the entoderm a cavity which is lined with a 

 vibratile epithelium. This cavity communicates with the outside by an 

 aperture which is almost opposite the shell, so that small molecules can 

 enter. The edges of the aperture may turn toward the outside and thus 

 act as a suction apparatus. Meanwhile the shell has assumed a regular 

 shape, aud has grown so much that it covers nearly half the surface of 

 the body. Soon a second aperture appears, through which the intestines 

 communicate with the outside ; this is the anas ; while the stomach now 

 becomes pear-shaped. Its larger part is turned forward, and on the 

 fourth or fifth day a diverticle appears on each side of the stomach ; 

 these are the two halves of the liver, in the sides of which small glob- 

 ules of oil may be seen which strongly refract the light. 



The study of the development of the American oyster, therefore, 

 teaches us these four things : 



1. That there is a gastrular stage caused by invagination. 



2. That the blastopore closes, aud that the anus and the latter do not 

 coincide. 



H. Mis. 08 57 



