DEVELOPMENT IN PLANARIANS. 2/9 



slip a new head will be regenerated, and if a fairly considerable 

 amount of the intestinal apparatus be contained in the slip, a new 

 pharynx will be formed. The intestinal apparatus, if it originally 

 consists of isolated parts, tends to become anastomosed into a 

 whole. A new head is formed about that portion of the intes- 

 tinal system which projects on the cut surface in the neighbor- 

 hood of the most developed portion of the nerve-cord (see Fig. 

 1 2, A). The new pharynx is formed posterior to the region of 

 least bodily compression, polarity being understood to be deter- 

 mined by the nervous system. If the lateral slip be cut so that 



FlG. 12. To illustrate the development of an individual from a lateral slip. 

 Magnification, 17 diameters. 



A. Piece isolated from the region lateral to the pharyngeal pocket. 



B. Two individuals developing from a lateral slip from the center of which a por- 

 tion of the nerve cord was removed. 



two or more regions will be formed in which the nervous and 

 intestinal apparatus of each section will be isolated from that of 

 the neighboring section, double or even triple individuals may be 

 formed. Fig 12, B, indicates a double individual of this kind. 



Having thus briefly considered the regeneration of new indi- 

 viduals from these typical isolated pieces, we may now take up, 

 first, the question of histogenesis during regeneration and, sec- 

 ond, that of the conditions determining the formation of definite 

 structures. 



HISTOGENESIS DURING REGENERATION. 



After removal of the piece the surface epithelium spreads out 

 so as to cover the cut area with a layer of epithelium like that 

 covering the embryo, but somewhat thicker than the latter (see 

 Fig. 3, g]. Stevens (op. r//.) gives an excellent account of this 

 process in Planaria lugnbris. Direct cell division restores the 

 epithelium to normal form. The cells regenerated on the dorsal 

 surface are not ciliated. 



