DEVELOPMENT OF THE OYSTER. 



Explanation of Plate XXXVII. 



Figure 1. Eggs two houi'S and seven minutes after fertilization. It is now perfectly spherical, with an external 

 membrane, ami the germinative vesicle is not visible. 



Figure 2. The same egg two minutes later. It is now elongated, one end is wider than the other, and a trans- 

 j);ireiit area at the broad end marks the point where the jiolar globules are about to a[)pear. At the opposite end 

 the external membrane is wrinkled by Maves which run from the uutiitiVe towai'd the formative pole in rai)id 

 succession for about tifteen seconds. 



Figure 3. The same egg two minutes later. 



Figure 4. The same egg two minutes later. The yolk has become pear-shaped. The polar globule has appeareil 

 at the formative pole, in the middle of the broad end of the pear, and the nutritive end of the egg is now less 

 granular than the formative end. 



Figure 5. Tlie same egg two minute.-j later. Tliree equidistant furrows have made their appearance, separating 

 it into a single mass at the nutritive pole, and two at the formative pole. At this stage the three masses are about 

 equal in size. 



Figure G. The same egg two minutes later. The first micromere, c, is now perfectly separated and smaller than 

 the second, h, and each is smaller than the macromere, a. 



Figure 7. The same egg one minute later. Both niicromeres are separated and spherical, as is also the 

 maciomere. Tiiis stage ends the first period of activity. 



Figure S. The same egg forty-five seconds later. The two micromeres have begun to fuse with each other, and 

 the seciond micromere, b, is also partially fused with the macromere, a. 



Figure 0. The same egg one minute later. The first micromere, c, has also begun to unite with the macromere. 



Figure 10. The same egg one minute later. The line between the second micromere and macromere has 

 disa])])eared, and the first micromere, c, now projects from one end of the elongated mass formed by the union of 

 the spherules a and b. 



Figure 11. The same egg three minutes later. The fusion of rt and b is now complete, and a large transparent 

 vesicle is now Aisible in the first micromere, c, and anotiicr in the compound mass a b. 



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



Figure 13. Another egg, about two minutes later. Tin's is the true resting 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 compound mass a b is elongared; the first 

 micromere, c, is well defined, and waves travel from tlie nutritive toward the formative ends of the two masses. 

 Two segmentation nuclei occupy the positions of tlie large vesicles of earlier stages. This stage is the beginning of 

 the second period of activity. 



I''igure IT). The same egg one minute later. The second niiciomere, b, is now well defined, as well as the first. 



Figure Ui. The same egg one minute later. This stage marks the end of the second jxM'iod of activity. The 

 foiiuative end of the egg is now occupied by four micromeres, two of which seem to be the products ol' the division 

 of the first micromere, «, and two of them the products of the second, b. 



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