DEVELOPMENT OF PARA VORTEX GEMELLIPARA 517 



c. Pharynx. Simultaneously with the differentiation of the 

 brain the anlage of the pharynx nausculature becomes marked 

 off from the former and from the remaining tissue by a closer 

 grouping of larger nuclei. This mass lies ventral and very 

 slightly posterior to the brain, but its boundaries are at first 

 so indefinite that, were it not for the simultaneous ingrowth of 

 the cells which form the internal pharyngeal epithelium, the 

 anlage would with difficulty be distinguished. 



Considerable attention has been paid by European observers 

 to the development of the phaiynx of the Turbellaria. Mattieson 

 in 1904 described the process in the Dendrocoele, Planaria torva, 

 in which the pharynx arises as a spherical mass of cells on the 

 ventral surface just beneath the primary ectoderm. While 

 the outer cells form the musculature of the organ, the internal 

 epithelium arises from a few large cells situated on the interior. 

 Against the inner end of the pharyngeal mass lie four or five 

 cells which in Mattieson's opinion are entodermic in nature. 

 No vestibule intervenes between the pharynx and mouth. 



Bresslau ('04) found that in the Rhabdocoele Bothromesosto- 

 mum personatum the internal epithelium likewise arises in loco. 

 In Mesostomiun ehrenbergi, a different condition exists; the 

 lining epithelium is derived from a rod of cells which grows 

 inward from the ectoderm and pierces longitudinally the mass 

 which eventually becomes the pharyngeal musculature with its 

 glands. After traversing the pharynx the ectodermic rod ac- 

 quires a lumen and gives rise at the inner end to what Bresslau 

 terms the inner pharngeal pouch lying between phaiynx and 

 yolk, while between the pharynx and mouth an outer pharyngeal 

 pouch has a similar origin. 



The main features in the development of the pharynx in Para- 

 vortex gemellipara agree with those described by Bresslau for 

 Mesostomum ehrenbergi. Until after the ingrowth of the epi- 

 thelial rod no definite grouping of nuclei occurs to mark the 

 origin of the pharyngeal bulb. The cells which are to take a 

 part in its formation lie loosely arranged in the mid-ventral 

 portion of the undifferentiated cell mass just beneath and pos- 

 terior to the brain. 



