250 FISHERIES OF THE UNITED STATES. [66] 



Ostrea Virginica, Ginelin. American Oyster — Continued. 



Figure 26. — The egg shown in Figure 24, fifty-five minutes later. The 

 macromere, a, is almost covered by the ectoderm, and the second micromere, 

 b, has divided into a number of spherules. At the growing edge, g, an ec- 

 toderm spherule is seen separating from the macromere. 



Figure 27. —A similar view of an egg twenty-seven hours after impregna- 

 tion. The macromere is almost covered by the ectoderm, to, and is not 

 visible in a side surface view. At g is an ectoderm spherule, which is sep- 

 arating from the macromere. 



Figure 28. — Optical section of the same egg; ec, ectoderm; en, macro- 

 mere, divided into two spherules. No segmentation cavity can be seen in 

 a normal egg at this or any of the preceding stages. 



Figure 29. — View of the nutritive pole of an egg a few hours older. 



Figure 30. — View of the formative pole of a still older egg. 



Figure 31. — Optical vertical section of a somewhat older egg,- figured with 

 the polar globule above and the ectoderm to the right. The egg is now flat- 

 tened from above downwards, and is disk-shaped in a surface view. The 

 macromere has given rise to a layer of larger granular cells, which are 

 pushed in so as to forma large cup-shaped depression. The more trans- 

 parent ectoderm, ec, now carries a few short cilia scattered, irregularly, and 

 the two layers are separated from each other by a segmentation cavity. This 

 figure is in Plate III. 



Figure 32. — Surface view, and 



Figure 33. — Optical section of the embryo at the first swimming stag^. 

 The ectoderm has folded upon the endoderm, so as to form a primitive diges- 

 tive cavity, with an external opening, g. The cilia of the velum have now 

 made their appearance around the area occupied by the polar globule. This 

 was not present, in the egg from which the figure was drawn, but it was seen 

 in other eggs, and is shown in a later stage of another embryo, Figure 6. 



Figure 34 aud Figure 35. — Two surface views of the emhryo shown in 

 Figure 32. 



Figure 36. — An older embryo, in the same position as Figures 32 and 33. 

 The external opening of the primitive digestn o tract has closed up, aud the 

 two valves of the shell have appeared in the place which it had occupied. 

 The endoderm has no connection with the exterior, and no central cavity 

 could be seen. 



Figure 37. — A somewhat older embryo, figured with its dorsal surface above. 

 There is a large, central, ciliated digestive cavity which opens externally 

 by the mouth m, which is almost directly opposite the primitive opening, 

 the position of which is shown by the shell, s. 



Figure 38. — A similar view of a still older embryo. The shell, s, has in- 

 creased in size, and the digestive tract has two openings, the mouth, m, 

 and the anus, an, which are very near each other on the ventral surface. 



Figure 39. — The opposite side of a still older embryo, in which the body 

 wall begins to fold under the shell, to form the mantle m. 



Figure 40. — Dorsal view of an embryo at about the same stage. 



Figure 41. — Dorsal view of an embryo at the stage shown in Figure 88, 

 with its valves extended ; rs, right valve of shell; Is, left valve of shell; an, 

 anus; a, anal papilla; ma, mantle; v, velum; b, body cavity; st, stomach. 



Figure 42. — View of left side of a still older embryo; i, intestine. Other 

 letters as in Figure 41. 



Figure 43. — Dorsal view of an embryo six days old, swimming by the 

 cilia of its velum. 



Figure 44. — View of right side of another embryo at the same stage; mu } 

 muscles ; I, liver. Other letters as in Figure 41. 



