2/4 CHARLES RUSSELL BARDEEN. 



I, C, may, with care, be kept alive until adult form is assumed; 

 but if mechanically injured they do not long survive and pieces 

 removed from them will not live. If embryos of the stage shown 

 in Fig. i, D, be cut in a region immediately anterior to the 

 pharynx both anterior and posterior pieces may sometimes be 

 kept alive. The anterior piece will develop a new pharynx, but 

 the posterior piece will not develop a new head, although it may 

 otherwise undergo full development, including the assumption of 

 the pigmentation characteristic of the adult. It seems probable, 

 although it cannot be taken as proved, that the failure of the 

 posterior piece to give rise to a new head is due to the absence 

 in the piece of well-formed nerve cords. If the cut be made near 

 the head or in older embryos a new head may be regenerated. 



In Dcndroccclum lactemn ! F. R. Lillie found that a new tail 

 may be formed by outgrowth from any transverse level behind 

 the region immediately back of the eyes, but a head only on a 

 cut surface made in the anterior third of the body. These re- 

 sults have been confirmed by E. Schultze ~ and by me. Lillie 

 ascribes the result to the cephalization of the nervous system in 

 D. lacteuni. It is interesting to note that at the period when the 

 central nervous system is " cephalized " in Planaria jnaculata 

 similar conditions prevail. 



REGENERATIVE DEVELOPMENT. 



For the sake of comparing embryonic with regenerative devel- 

 opment we may consider briefly the regeneration of new individ- 

 uals from cross pieces taken from in front of the pharynx, from 

 tail pieces and from slips removed from a region lateral to the 

 median line. 



Cross Pieces. There is a close similarity between the develop- 

 ment of a new individual from a cross piece cut from in front of 

 the pharynx and development in the embryo. In the cross piece 

 the pharynx develops on the posterior ventral surface, with rela- 

 tions to the axial gut similar to that borne by the pharynx to the 

 yolk cavity in the embryo (see Fig. 8). The head and tail re- 



1 " Notes on Regeneration and Regulation in Planarians," American Journal of 

 Physiology, Vol. VI., p. 128, 1901. 



2 " Aus dem Gebiete der Regeneration bei Turbellarien," Zeitschrift fur wissen- 

 schaftliche Zoologie, Vol. LXXII., p. I, 1902. 



