DEVELOPMENT OF THE OYSTER. 



Explanation of Plate XXXYIII. 



ri,i;ure 17. The same ogg two luinutos later, at the coiiimencement of the third period of rest. The second 

 iDicroiiiere, b, has again oeyuii to fuse witli the macroinere, a. * 



Fi'4ure 18. The same egg- three miuutes and thirty seconds later. The second micromere is no longer separated 

 from the niacroniere, and the mass a h, formed by their union, is nearly spherical. 



Figure 19. Tlie same egg two miuutes and a half later, at the end of the third period of rest, viewed at right 

 angles to Figure 18. 



Figure 20. The same eg^;^ thirteen minutes later, and in the same position as Figure 18. The spherule, c, of 

 Figure 10, has divided into two, and the second micromere, b, has become prominent, so that there are live 

 micromeres at the formative pole. 



Figure 21. Tiie same egg one minute later, and in the same i>osition as Figai'e 10. 



Figure 22. The same egg in the position of Figure 20, fifteen miuutes later than Figure 21, and in the fourth 

 period of activity. Thci-e are now seven micromeres at the formative pole, six on one side of the polar globules 

 and one, the second micromere, b, on the other. 



Figure 23. The same egg twenty-one minutes later, viewed from the side opposite the second micromere. The 

 cells, which have beeu formed by the division of the micromeres of the stage 19, now form a layer, the ectoderm, 

 which rests, like a cap, on the macromere, a. 



Figure 21. The same egg five hours and fifteen minutes later, in the same position as Figure 22, but not quite 

 as much magnified. On one side the polar globule is still separated from the macromere, «, by a single spherule — 

 the second micromere, b. Opposite this the growing edge, g, of the ectoderm is spreading still farther down over 

 the macromere. At the point g, and at four other points, are pairs of small cells, which have evidently been formed 

 by the division of the larger spherules. 



Figure 25. Another egg at about the same stage. 



Figure 26. The egg shown in Figure 21, fifty-five minutes later. The macromere, a, is almost covered by the 

 ectoderm, and the second micromere, b, has divided into a uiimber of spherules. At the growing edge, g, an 

 ectoderm spherule is seen separating from the macromere. 



Figui'e 27. A similar view of an egg twenty-seven hours after impregnation. The macromere is almost covered 

 by the ectoderm, e c, and is not visible in a side-surface view. At g is an ectoderm spherule, which is sepai'atiug 

 from the macromere. 



Figure 28. Optical section of the same egg; e c, ectoderm; c n, macromere, divided into two spherules, l^o 

 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 iiolar globule aboA'e and the 

 ectoderm to the right. The egg is now flattened from above downward, and is disc-shaped in a surface-view. 

 The macromere has given rise to a layer of larger granular cells, which are i)ushed in so as to loim a large cup- 

 shaped depression. Tlie more transparent ectoderm, e c, 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 XXX IX. 



Figure 32. Siu-face-view of the embrvo at the first swimming stage. 



