DEVELOPMENT OF THE OYSTER. 



Explanation of Plate XXXIX. 



Figure 33. Oi)tic.al section of tbe cmbiyo at tlie first swimniing stage. The ectoderm lias folded upon the 

 eiuloderm, so as to form a primitive digestive cavity, with an external ojiening, f/. The cilia of the velum have now 

 made their appearance around the area occupied by the polar globule. This was not jiresent in the egg from which 

 the figure was drawn, but it was seen in other eggs, and is shown in a later stage of another embryo, Figure 3G. 



Figures oJ: aud 3.5. Two surface- views of the embryo shown in Figure 32. 



Figure 36. An older embryo, in the same position as Figures 32 and 33. The external opening of the primitive 

 digestive tract has closed up, and tbe two valves of the shell have appeared in the place which it had occupied. 

 The endoderni has no connecticjn with the exterior, and no central cavity could be seen. 



Figure 37. A somewhat older embryo, figured with its dorsal surface above. Tliere is a large, central, ciliated 

 digestive cavity wlueli 0])ens externally by the mouth, iii, which is almost directly oiiposite the i)rimitive oi^ening, 

 the position of which is shown by the sliell, s. 



Figure 38. A similaa- view of a still older embryo. The shell, a, has increased in size, and the digestive tract 

 lias two openings, the mouth, w, and the aims, a », wliich are very near each other on the ventral surface. 



Figure 39. The opposite side of a still older euibryo, in which the body-wall begins to fold under the shell, to 

 form the mantle, m. 



Figure 40. Dorsal view of an embryo at about the same stage. 



Figure 41. Dorsal view of an embryo at the stage shown in Figure 38, with its valves extended ; r s, right valve 

 of shell; I s, left valve of shell; « n, anus; a, anal papilla; m a, mantle; v, velum; h, body-cavity; s t, stomach. 



Figure 42. View of left side of a still older embryo; i, intestines. Other letters as in Figure 41. 



