250 MEAD. [Vol. XIII. 



The division of the four cells lying at the vegetative pole 

 {A^, B^ C^, D^ forms a definitive, cross-shaped, entoderm plate of 

 eleven cells. 



The trochophore consists of about two hundred cells when 

 the paratroch is differentiated, while the mesoblast bands, at 

 this time, are made up of four cells each. 



III. Formation and Elongation of the Trunk. 



a. Shifting of areas on the lower hemisphere. — The present 

 chapter will consider cleavage only as a means of orientation 

 in following the movements of embryonic areas, during the 

 metamorphosis of the spherical one-layered trochophore into 

 the elongated three-layered larva. 



The prototroch naturally divides the trochophore into an 

 anterior umbrellar and a posterior subumbrellar region. Upon 

 the latter the more important shifting of areas occurs, involv- 

 ing the somatic plate, colored blue ; the mesoderm, red ; the 

 entoderm, stippled; and the rest of the ectoderm, untinted. 

 At a stage when the mesoderm cells are still at the surface 

 (Figs. 38, 40-42), they, together with the entoderm cells, 

 occupy a relatively large area on the subumbrella, even larger 

 than that of the somatic-plate cells. 



The mesoderm sinks into the segmentation cavity, and the 

 somatic plate, by spreading out, comes to lie next to the pos- 

 terior edge of the entoderm plate. In the median line the 

 point of meeting of the two plates is about 90° from the proto- 

 troch (text Fig. VII). The eleven entoderm cells begin to invagi- 

 nate in exactly the same manner, and consequently the surface 

 area of the entoderm plate gradually diminishes until it finally 

 disappears. Since the cells all invaginate at a uniform rate, 

 the pattern of the plate on the surface remains nearly the 

 same in shape, though constantly diminishing in size. The 

 blastopore closes from all directions at once, and the stomodaeum 

 is formed where the entoderm cells were last seen on the sur- 

 face, i.e., about 30° behind the prototroch in the mid-ventral 

 line (later, however, when the oesophagus is formed, the mouth 



