many of the surface cells have acquired cilia and the embryos 

 begin to roll around on the bottom of the dish. 



An hour or two later the apical cilia make their appearance. 

 They at first are not much longer than the others, and do not 

 seem to be very numerous. Because of their motion and tend- 

 ency to bunch together they are hard to count, but only four or 

 five seem to be present at this early stage. They grow quite 

 rapidly until they are nearly as long as the diameter of the 

 embryo and increase in number until a considerable bunch is 

 formed. 



About this time the embryo begins to elongate slightly in the 

 axis roughly corresponding to the direction of the apical tuft 

 of cilia and the embryo begins to swim freely in the water. The 

 motion is not very rapid, and is at first rolling, but as the apical 

 cilia elongate the embryo begins to swim in definite lines, always 

 with the apical cilia pointing forward. In swimming the em- 

 bryo varies its direction almost constantly, and continually 

 rotates on its longitudinal axis. The direction of the rotation 

 may be changed from time to time. 



Sections at this stage (fig. 33) show two pouches formed by 

 the invagination of the surface layer of cells. On 

 what is to become the dorsal portion of the animal, 

 nearly opposite the apical cilia, is the larger of these two 

 pouches (sg.). It is composed of large cells that are con- 

 tinuous with the surface cells. This is the shell gland. It 

 soon spreads out and grows down on the sides to form 

 the lobes of the mantle and to secrete the shell. The other 

 invagination (ar.) is somewhat smaller than the one just 

 described, is composed of smaller cells, and is situated on the 

 ventral side. Like the other, this is continuous with the sur- 

 face layer of cells. It has been formed apparently partially by 

 the pushing in of surface cells, and partially by the division and 

 separation of cells on the inside of the embryo. This is the first 

 appearance of the alimentary canal, and probably represents a 

 combined archenteron and stomodaeum. The inner ends of the 

 shell gland and archenteron lie very close to each other and may 

 for a time be in contact. With the spreading out of the shell 

 gland, which is accomplished in about 18 or 20 hours after the 

 egg is fertilized, the embryo elongates decidedly (fig. 34) and 



56 



