The Development of Phascolosoma. 115 



a transitory phenomenon, and there are no external evidences of 

 division into somites. The mesoderm soon becomes split into somatic 

 and splanchnic layers, at first in the anterior segmented portion, and 

 then further back. 



(2) Dissolution of the Prototroch. Clusters of yolk 

 granules can be seen in the coelom immediately behind the prototroch, 

 even before the splitting of the mesoblast has been completed. These 

 granules come from the prototroch cells, which degenerate on their 

 inner side, while their nuclei and the superficial portion of their 

 •cytoplasm remain for a considerable time intact. 



The retractor muscles now begin to contract intermittently, and 

 the introvert (including the entire prototrochal region) is involuted 

 into the coelom, as early as the splitting of the latter permits. The 

 interior extremity of a contracted individual is therefore the region 

 of the middle sphincter, and it is here that the accessory retractors 

 are inserted. Each act of introversion is completed by the con- 

 traction of all these sets of muscles. 



Loose masses of yolk granules break oif from the inner degene- 

 rating side of the prototroch cells, and during the repeated intro- 

 versions are left within the coelom of the trunk. It is readily seen 

 that the prototroch cells, at each retraction, are at first compressed 

 between the apical plate and the rigid line of the sphincter muscle, 

 -and an instant later are turned inside out within the coelom. At the 

 end of this extraordinary process, the entire substance of the pro- 

 totroch has been transformed into yolk, and been cast into the 

 coelom. The degenerating nuclei of all these cells are also found 

 lying within the coelomic fluid. The nuclear membrane of each 

 contains a homogeneous deeply-staining mass, in which there is 

 usually a large unstained vacuole. 



(3) Overgrowth. There is for a short time not only a dorsal 

 but also a ventral interruption of the prototroch. The cells of the 

 apical plate spread backward ventrally to the stomodaeum, and the 

 dorsal cord unites the apical plate to the ectoderm of the dorsal 

 part of the trunk. Thus the prototroch cells occupy two isolated 

 areas, one on each side of the posterior part of the introvert. This 

 region, during the course of the two or three hours which follow, 

 is gradually vacated by the degenerating prototroch cells, and over- 

 grown by the adjacent ectoderm. 



This growth takes place chiefly from the dorsal ectoderm in 

 front of the region of the postoral circlet. The posterior part of 



8* 



