DEVELOPMENT IN PLANARIANS. 



265 



B the permanent pharynx is shown near the posterior end of the 

 piece. This individual seems to have been a small one at the 

 time of metamorphosis. In C and D the development of the tail 

 posterior to the pharyngeal region is shown. There is a tem- 

 porary anastomosis of the posterior rami behind the pharyngeal 

 pocket. This anastomosis later disappears, as shown in Fig. I, 

 D. The head is meanwhile developed, as shown in the figures, 

 anterior to the primitive yolk cavity. 



The relations of the head, pharyneal and tail regions to the 

 primitive yolk cavity of the embryo are also represented in Fig. 

 2. In B the anlage of the permanent pharynx is shown. The 



pkp 



FIG. 2. Sagittal sections in the median line of embryos of various stages of 

 development. Magnification, 57 diameters. 



A. Section through an embryo which is becoming filled with yolk cells. 



B. Section through an embryo in which both the embryo-pharynx and the anlage 

 of the permanent pharynx are seen. 



C. Section through an embryo in which the two posterior extensions of the intes- 

 tinal cavity are uniting behind the pharyngeal pocket. 



D. Section through an embryo in which the tail region is well developed and 

 the anlage of the head is well marked. Owing to the longitudinal contraction 

 caused by the killing fluid the pharynx has been partly forced out from the pharyn- 

 geal pocket, e. ph., Embryo pharynx, ph. p., Pharyngeal pocket. 



base of the pharynx extends obliquely from the region of the 

 embryo-pharynx to the ventral wall. In C the base of the phar- 

 ynx still extends obliquely from the dorsal to the ventral walls. 

 The embryo-pharynx has disappeared. A tail region posterior 

 to the pharynx and a head region anterior to the yolk cavity are 



