[65] 



FISHERIES OF THE UNITED STATES. 



Ostrea Virginica, Gmelin. American Oyster — Continued. 



Figure 10. — The same egg oue minute later. The line between the sec- 

 ond micromere and raacromere has disappeared, and the first micromere, c, 

 now projects from one end of the elongated mass formed by the union of 

 the spherules a and h. 



Figure 11. — The same egg three minutes later. The fusion of a and h is 

 now complete, and a large transparent vesicle is now visible in the first 

 micromere, c, and another in the compound mass, ab. 



Figure 12. — The same egg two minutes and thirty seconds later. 



Figure 13. — Another egg, about two minutes later. This is the true rest- 

 ing stage, at the end of the second period of rest. The two vesicles have 

 become irregular. The remains of an external membrane adhere to one 

 side of the egg. 



Figure 14. — The same egg seven minutes later than Figure 13. The com- 

 pound mass, a and h, is elongated, the first micromere, c, is well defined, 

 and waves travel from the nutritive towards the formative ends of the two 

 masses. Two segmentation nuclei occupy the positions of the large vesicles 

 of earlier stages. This stage is the beginning of the second period of ac- 

 tivity. 



Figure 15. — The same egg one minute later. The second micromere, h, is 

 now well defined, as well as the first. 



Figure 16. — The same egg one minute later. This stage marks the end 

 of the second period of activity. The formative end of the egg is now oc- 

 cupied by four micromeres, two of which seem to be the products of the 

 division of the first micromere, c, and two of them the products of the sec- 

 ond, b. 



Figure 17. — The same egg two minutes later, at the commencement of the 

 third period of rest. The second micromere, b, has again begun to fuse 

 with the macromere, a. 



Figure 18. — The same egg three minutes and 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, and in the same posi- 

 tion as Figure 18. The spherule, c, of figure 19 has divided into two, and the 

 second micromere, b, bas become prominent, so that there are five micro- 

 meres at the formative j)ole. 



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, and 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 twenty-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 ecto- 

 derm, 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 so much magnified. On one side the 

 polar globule is still separated from the macromere, a. by a single spherule — 

 the second micromere, b. Opposite this the growing edge, jf, oftheeco- 

 derm is spreading still fiirthcr 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 large spherules. 



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

 228a 5 



