FISHERIES OF THE UNITED STATES. 47 



cupy the positions of the large vesicles of earlier stages. This 

 stage is the beginning of the second period of activity. 



Figure 15. — The same egg one minute later. The second mi- 

 cromere, 6, is now well detined, as well as the first. 



Figure IG. — The same egg one minute later. This stage marks 

 t'he end of the second period of activity. The formative end of 

 the egg is now occupied l)y four micromeres, two of which seem 

 to be the iiroducts of the division of the first micromere, c, and. 

 two of them the products of the second, b. 



Figure 17. — The same egg two minutes later, at the commence- 

 ment of the third period of rest. The second micromere, h, has 

 again begun to fuse with the macromere, a. 



Figure 18. — The same egg three minutes aud thirty seconds 

 later. The second micromere is no longer separated from the 

 macromere, and mass, a and b, formed by their union is nearly- 

 spherical. 



Figure 19. — The same egg two minutes and a half later, at the 

 end of the third period of rest, viewed at right angles to Figure 18. 



Figure 20. — The same egg thirteen minutes later, aud in the 

 same position as Figure If^. The spherule, c, of figure 19 has 

 divided into two, and the second micromere, b, has become promi- 

 nent, so that there are five micromeres at the formative pole. 



Figure 21. — The same egg one minute later, and in the same 

 position as figure 19. 



Figure 22. — The same egg in the position of figure 20, fifteen 

 minutes later than figure 21, aud in the fourth period of activity. 

 There 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 tweuty-one minutes later, viewed 

 from the side opposite the second micromere. The cells which 

 have been 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 24. — The same egg five hours and fifteen minutes later, 

 in the same position as figure 22, but not quite as much magni- 

 fied. On one side the polar globule is still separated from the 

 macromere, «, by a single spherule — the second micromere, 6. 

 Opposite this the growing edge, (/, 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 siiherules. 



Figure 25. — Another egg at about the same stage. 



Figure 20. — The egg shown in figure 24, fifty-five minutes later. 

 The maci'omere, 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 ectoderm spherule is seen separating 

 from the macromere. 



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

 impregnation. The macromere is almost covered by the ecto- 

 derm, cc, and is not visible in a side surface view. At g is an 

 ectoderm spherule, which is separating from the macromere. 



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

 macromere, divided into two spherules. No segmentation cavity 

 can be seen in a normal egg at this or any of the preceding 

 stages. 



