All of these stages 



At an early stage of segmentation it looks something like Fig. 3, the two polar 

 globules being still seen attached to the egg. Fig. 4 shows the 

 result of this rapid cell division, and F'ig. 5 a little later period 

 than that shown in Fig. 4. — 27 hours 

 after impregnation and just previous 

 to the swimming stage shown in Fig. 6. 

 At this period, thirty hours after fer. 

 tilization, the embryo moves rapidly 

 through the water by means of the cilia 

 i-ig, 5. or hair-like growths shown in the figure, 



can be readily seen under the microscope and were observed repeatedly by the 

 writer in his seaside laboratory. I igure 7 shows an oys- 

 ter embryo at a considerably later date. 2 to 4 days after 

 fertilization, with its delicate transparent shell showing, 

 the alimentary canal and the mouth. When the oyster 

 embryos are at the stage shown in Fig. 6, they crowd to 

 the surface in large numbers. It is at this time that thev 

 are explosed to the greatest dangers, to the danger of 

 being eaten by small fish, danger from change of air 

 temperature, ^:c. Hence the value of artificial fertiliza- 

 tion and subsequent care in the laboratory, in 

 which process they need not be consigned to 

 ■the outside waters until they have passed the 

 critical period alluded to. It is important, 

 therefore, that this process should be resorted 

 to every season until the introduced species 

 has obtained a foothold. 



Later, when they have acquired their deli- 

 cate shells, they sink to various depths and are 

 less liable to fatalities. It is not, however, 

 until they have swum about for from 6 to 8 

 days that they settle on a piece of clean shell, or piece of crockery, a brick, old 

 shoe or rubber, clean wood or rock, anything in fact that is not slimy and there 

 become fastened as sj^at (Fig. 8.) never to move from that place of their own ac- 

 cord. The spat of the eastern oyster when first formed is about 

 1-80 inch in diameter. They are hardly marketable before they 

 are four years old and maj' under favorable conditions live for 20 

 years or more. The spat are very apt to form in clusters on other 

 shells. Tonging brings these clusters to the surface and the oys- 

 ters are separated, the larger specimens are marketed, the smaller 

 individuals being placed upon private beds to attain greater size. Hence we see in 

 markets and in restaurants single oysters and not clusters, nor are we apt to see the 

 original object to which each oyster was first attached as a spat. 



Biological Laboratory, Nov. 20, 1898 



