DEVELOPMENT OF PARA VORTEX GEMELLIPARA 519 



notice of these cells will be taken when the development of the 

 intestine is considered. 



In an older embryo (fig." 37) the muscular portion of the pharynx 

 has become sharply defined, although no external epithelium 

 has yet developed. The nuclei have not migrated farther toward 

 the periphery than the point at which they were observed in the 

 preceding stage. A considerable enlargement of the anterior 

 and posterior cells of the epithelial rod, has occured. No blind 

 sac, however, is formed, as in Mesostomum, at either end of the 

 pharynx. Indeed, no lumen appears in the pharyngeal apparatus 

 until shortly before the embryo leaves the mother's body. 



If at this stage the pharynx and associated tissues of Paravor- 

 tex gemellipara be compared with the corresponding stage in 

 Mesostomum ehrenbergi it is evident that the development of 

 the pharynx in the former lags behind that in the latter. Meso- 

 stomum shows a well defined epithelium bounding a large 

 cavity, the latter consisting of an internal and external pocket 

 communicating by a tube through the pharynx. Bresslau 

 considered the interior pocket as the esophagus, whose cells 

 differed in point of origin from those later forming the intestine. 

 The homologue of this pocket in P. gemellipara is the group of 

 large cells just posterior to the pharynx. 



In the cross section of this organ represented in figure* 38 an 

 advance is shown in the presence of an external epithelium. 

 A thin layer of plasma containing a few small flattened nuclei 

 covers the pharyngeal- bulb. Bresslau derived this tissue from 

 the outermost portion of the pharyngeal musculature anlage in 

 Mesostomum; but in Paravortex the indications point to a 

 mesenchymatous origin, such as von Graff ascribed to it in other 

 Turbellaria. When one remembers that in either case the cells 

 constituting this membrane arose in the undifferentiated cell 

 mass the point resolves itself into one of time. 



Concerning the development of the radial, circular and longi- 

 tudinal fibers which arise in the pharyngeal musculature nothing 

 has been observed in the young of Paravortex gemellipara while 

 still in the mother's body. 



