48 FISHERIES OF THE UNITED STATES. 



Figure 29. — View of the nutritive pole of an egg a few hours 

 older. 



Figure 30. — View of the formative pole of a still older egg. 



Figure :51. — Optical vertical section of a somewhat older egg, 

 figiu'ed with the polar globule above and the ectoderm to the 

 right. The egg is now flattened from above downwards, and is 

 disk-shaped in a surface view. The macromere has given rise to 

 a layer of larger granular cells, which are flushed in so as to form 

 a large cup-shaped depression. The more tTauspareut ectoderm, 

 ec, now carries a few short cilia scattered irregularly, and the two 

 layers are separated from each other by a segmentation cavity. 

 This figure is in plate III. 



Figure 32.— Surface view, and 



Figure 33. — Optical section of the embryo at the first swimming 

 stage. The ectoderm has folded upon the endodcrm, so as to form 

 a primitive digestive cavity, with an external opening, (j. The 

 cilia of the velum have now made their appearance around the 

 area occupied by the polar globule. This was not iiresent 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 6. 



Figure 34 and figure 35. — 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 the two valves of the shell have appeared in 

 the place which it had occupied. The endoderm has no connec-' 

 with the exterior, and no central cavity could be seen. 



Figure 37. — A somewhat older embryo, figured with its dorsal 

 surface above. There is a large, central, ciliated digestive cavity 

 which opens externally by the mouth, )ii, which is almost directly 

 opposite the primitive opening, the position of which is shown by 

 the shell, «. 



Figure 38. — A similar view of a still older embryo. The shell, 

 8, has increased in size, and the digestive tract has two openings, 

 the mouth, m, and the anus, an, which are very near each other 

 on the ventral surface. 



Figure 39. — The opposite side of a still older embryo, 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 Fig- 

 ure 38, with its valves extended ; rs, right valve of shell ; Is, left 

 valve of shell; cm, anus; a, anal j)apilla ; ma, mantle; v, velum; 

 6, body cavity; si, stomach. 



Figure 42. — View of lelt side of a still older embryo ; ?, intestine. 

 Other letters as in Figure 41. 



Figure 43. — Dorsal view of au emln-yo six days old, swimming 

 by the cilia of its velum. 



Figure 44. — View of right side of another embryo at the same 

 stage; mu, muscles; I, liver. Other letters as in Figure 41. 



Figure 48. — The seminal fluid of a rijte male oyster, mixed with 

 water, and seen with a power of 80 diameters. Zeiss, a. 2. 



Figure 49. — Fluid from the ovary of a ripe female oyster, seen 

 with the same magnifying power. 



Figure 50. — Seminal fluid of a ripe male oyster, magnified 500 

 diameters. 



