DYNAMICS — MORPHOGENESIS AND INHERITANCE 423 



entiation has occurred behind this surface and a single median 

 eye has developed. If differentiation proceeded far enough, 

 such a head would undoubtedly be teratomorphic. This form 

 of head develops in a week or ten days, but after two weeks or 

 more, even though kept in the anesthetic, further development 

 usually occurs, resulting in a form with overdevelopment of the 

 median region like figure 23, or sometimes in a head of normal or 

 nearly normal form (fig. 24) . Figure 23 shows the same type of 

 head as figures 19 and 20. This head-form is, in fact, characteris- 

 istic of conditions which inhibit head-development at first, but 

 permit some degree of acchmation. 



Figures 23 and 24 show another interesting feature. In the 

 earlier stages these heads posses a single median eye, but when 

 acclimation and further development take place, two more eyes 

 appear in normal position, so that the head finally possesses three 

 eyes. This must mean that in the earlier development the median 

 head region is so far inhibited that conditions determining differ- 

 entiation of eyes exist only at the median line. The region 

 between the eyes of normal heads is completely inhibited. Later, 

 when acclimation occurs, the median regions develop further 

 and eyes and nerves develop in normal relations. The less 

 inhibited and more completely acclimated heads approach nor- 

 mal form, except as regards the median eye (fig. 24), while less 

 complete acclimation gives forms like figure 23. These latter, 

 however, if returned to water, gradually approach normal pro- 

 portions. 



Secondary transformation of teratomorphic heads. The terato- 

 morphic head apparently represents a rather unstable equilib- 

 rium, for its frequency is usually much lower than that of 

 other head-forms, and heads which at first develop as terato- 

 morphic not uncommonly undergo transformation after a week 

 or two into teratophthalmic forms. This transformation may 

 occur in water at room temperature, as well as in cases of accli- 

 mation to inhibiting external conditions. Figures 25 to 28 

 show heads which were originally teratomorphic heads re- 

 sembling those of figures 5 and 6. The transformation involves 

 changes in both the cephalic lobes and the eyes, besides the internal 



