CONTROL OF MORPHOGENESIS IN PLANARIA. 319 



rence has been noted by various authors, though no one has 

 previously attempted to determine the conditions of their ap- 

 pearance. They are of various kinds, but in most cases fall into 

 one of the following groups: differences in size or asymmetries 

 of position (Figs. 9-11) ; approach to the median line (Figs. 1 1-12) 

 partial union of pigment areas (Figs. 13-19); single median pig- 

 ment areas with double sensory areas (Figs. 20-21); single 

 median eyes (Fig. 22) ; three eyes, one median, others right and 

 left (Fig. 23). In all of these cases the head may be entirely 

 normal otherwise, though the more extreme types of abnormal 

 eyes (e. g., Figs. 20-22) usually occur in heads of relatively small 

 size and slow rate of development. 



4. Teratomorphic. (Figs. 24-27.) Very frequently, however, 

 the head itself is abnormal in form, the median anterior region 

 being incompletely developed or absent and the auricles appear- 

 ing on the anterior end of the head and in various degrees of 

 approach to the median line: they may be separated by a con- 

 siderable interval (Figs. 24, 25) , or partially (Fig. 26) or completely 

 united in the median line (Figs. 27). As soon as the new heads 

 become pigmented the unpigmented auricular areas are clearly 

 visible and in such cases as Figs. 26 and 27 it is these areas which 

 enable us to identify with certainty the auricles. Teratomorphic 

 heads almost invariably possess only a single median eye, or 

 they develop a single median eye first and later two more sym- 

 metrically placed eyes (Fig. 23) ; they are always of small size 

 and develop slowly. 



5. An ophthalmic. (Figs. 28-34.) These pieces show a dis- 

 tinct outgrow r th of new tissue at the anterior end, varying widely 

 in form, but always without eyes. This outgrowth is well 

 innervated and its motor activities resemble those of a head. 

 Sometimes the partially or wholly fused auricles appear anteriorly 

 in the median line on the outgrowth (Figs. 28, 29), as in the 

 extreme type of teratophthalmic pieces (Figs. 26, 27), but 

 usually the outgrowth shows no visible differentiations charac- 

 teristic of a head. In shape the outgrowth ranges from a broad 

 and blunt form like Figs. 30 and 31 to a very slender form of 

 varying length, often almost as long as the remainder of the 

 piece (Figs. 32-34). These pieces always give rise to a new 



