220 TORREY 



TEXT-FIGURE 9. 



Optical sections of various stages to illustrate the history of the ectomesoblast 

 cells. A, section at the same plane as Fig. 10, A, but of a slightly older 

 stage, showing the teloblast-like budding of the right (^etnr) and left {^enil) ecto- 

 mesoblast, and also the migration of the median (<?w;;/) ectomesoblast. The Afs 

 have each budded off a small entoblast cell [e, e). B, section at the same plane 

 as Fig. 10, B, but of an older embryo, le, lateral ectomesoblast cells from the 

 right and left inlergirdle regions of the first quartet ; re, rudimentary cells sunk 

 into the entoblast cells and degenerating. C, section from the left side. The 

 blastopore {bl) has migrated toward the prototroch. The M's have migrated 

 laterally leaving behind the small entoblast cells [e, e). An ectomesoblast cell, a, 

 in-sinking from the first quartet, probably from the b arm of the cross. D, longi- 

 tudinal section of a considerably older stage. The small entoblast cells, derived 

 from the M^s, are seen in the posterior end of the enteron. The ectomesoblast is, 

 in part, becoming transformed into muscles. 



