248 FISHERIES OP THE UNITED STATES. [64] 



Ostrea Virginica, Gmelin. American Oyster. 



BIOLOGY. 



SERIES OF ILLUSTRATIONS OF THE EMBRYOLOGY OF THE AMERICAN 

 OYSTER, PREPARED FOR THE MARYLAND FISH COMMISSION BY DR. W. 

 K. BROOKS,'- PH. D., OF JOHNS HOPKINS UNIVERSITY, BALTIMORE. 



Explanation of the figures. 



Unless the contrary is stated, the figures are drawn with a magnifying 

 power of 250 diameters: Zeiss. F. 2, but it was necessary to amplify the 

 sketches considerably in order to reproduce, by the process of photo- 

 engraving, the features which this magnifying power rendered visible, and 

 the figures as they are produced are of about twice the diameter of cam- 

 era sketches made with the same magnifying power. 



The first thirty-two figures show the process of segmentation. Figure 

 1 is an egg at the end of the first period of rest ; Figures 2, 3, 4, 5, 6, and 

 7, the changes during the first period of activity; Figures 8, 9, 10, 11, 12, 

 and 13, the changes during the second period of rest; Figures 14, 15, and 

 16. those which take place during the second period of activity; 17, 18, and 

 19, those Avhich take place during the third period of rest; 20 and 21, dur- 

 ing the third period of activity ; 22, during the fourth period of activity ; 

 23, during the fifth period of activity, and the remaining figures show more 

 widely separated stages. In all the figures of segmentation, except 20, 3", 

 and 31, the formative pole is above and the nutritive pole below. 



Figure 1. — Eggs two hours and seven minutes after fertilization. It is 

 now perfectly spherical, with an external membrane, and 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 transparent area at the broad end marks 

 the point where the polar globules are about to appear. At the opposite 

 end the external membrane is wrinkled by waves which run from the nu- 

 tritive towards the formative pole in rapid succession for about fifteen sec- 

 onds. 



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 appeared at the formative pole, in the mid- 

 dle of the broad end of the pear, and the nutritive end of the egg is now 

 less granular than the formative end. 



Figure 5. —The same egg two minutes later. Three equidistant furrows 

 have made their appearance, separating it into a single mass at the nutri- 

 tive pole, and two at the formative pole. At this stage the three masses 

 are about equal iu size. 



Figure 6. — The same egg two minutes later. The first micromere, c, is 

 now perfectly separated, and smaller than the second, b, and each is smaller 

 than the macromere, a. 



Figure 7. -The same egg one minute later. Both micromeres are sepa- 

 rated and are spherical, as is also the macromere. This stage ends the 

 first period of activity. 



Figure 8. — The same egg forty-five seconds later. The two micromeres 

 have begun to fuse with each other, and the second micromere, b, is also 

 partially fused with the macromere, a. 



Figure 9. — The same egg one minute later. The first micromere, c, has 

 also begun to unite with the macromere. 



