[23] FISHERIES OF THE UNITED STATES. 207 



the greater part of the surface of a single large spherule, and the series 

 of figures shows that the latter is the spherule which is below in Fig. 

 G. This spherule now divides up into a layer of cells, and at the same 

 time the egg, or rather the embryo, becomes flattened from above down- 

 ward, and assumes the shape of aflat oval disk. Figs. 29 and 30 are views 

 of the upper and lower surface of the embryo at about this time. In a 

 sectional view, Fig. 31, it is seen to be made up of two layers of cells; 

 an upper layer of small transparent cells, ec, which are to form the outer 

 wall of the body, and which have been formed by the division of the 

 spherules which occupy the upper end of the egg in Fig. 25, and a 

 lower layer of much larger, more opaque cells, g, which are to become the 

 walls of the stomach, and which have been formed by the division of the 

 large spherule a of Fig. 25. 



"This layer is seen in the section to be pushed in a little towards the 

 upper layer, so that the lower surface of the disk-shaped embryo is not 

 flat, but very slightly concave. This concavity is destined to grow 

 deeper until its edges almost meet, and it is the rudimentary digestive 

 cavity. A very short time after this stage has been reached, and usu- 

 ally within from two to four hours after the eggs were fertilized, the 

 embryo undergoes a great change of shape, and assumes the form which 

 is shown in three different views in Figs. 32, 33, 34, and 35. 



"A circular tuft of long hairs or cilia has now made its appearauce 

 at what is thus marked as the anterior end of the body, and as soon as 

 these hairs are formed they begin to swing backwards and forwards in 

 such a way as to constitute a swimming organ, which rows the little 

 animal up from the bottom to the surface of the water, where it swims 

 around very actively by the aid'of its cilia. This stage of development, 

 Fig. 32, which is of short duration, is of great importance in raising 

 the young oysters, for it is the time when they can best be siphoned off 

 into a separate vessel and freed from the danger of being killed by the 

 decay of any eggs which may fail to develop. On one surface of the 

 body at this stage, the dorsal surface, there is a well marked groove, 

 and when a specimen is found in a proper position for examination, the 

 opening into the digestive tract is found at the bottom of this groove. 

 Fig. 33 is a sectional view of such an embryo. It is seen to consist of a 

 central cavity, the digestive cavity, which opens externally on the dor- 

 sal surface of the body by a small orifice, the primitive mouth, and which 

 is surrounded at all points, except at the mouth, by a wall which is dis- 

 tinct from the outer wall of the body. Around the primitive mouth these 

 two layers are continuous with each other. 



" The way in which this cavity, with its wall and external opening, 

 has been formed will be understood by a comparison of Fig. 33 with 

 Fig. 28. The layer which is below in Fig. 28 has been pushed upwards 

 in such a way as to convert it into a long tube, and at the same time 

 the outer layer has grown downwards and inwards around it, and 

 has thus constricted the opening. The layer of cells which is below in 



