DYNAMICS — MORPHOGENESIS AND INHERITANCE 417 



cannot be determined with certainty because of the absence of 

 well-defined landmarks in this region, but the conditions found 

 in the teratomorphic heads make it probable that at least in 

 the more extreme teratophthalmic forms the median region is 

 more or less inhibited throughout the whole length of the head. 



The series of dotted lines in figure 4 are intended to represent 

 in a diagrammatic way approximately the regions inhibited in 

 various degrees of teratophthalmia. At the level of the ganglia 

 the degree of median inhibition can of course be determined with 

 relative exactness by the degree of approximation of the eyes 

 and in section of the ganglia. As regards the preganglionic 

 region, however, the lines in figure 4 are merely suggestive. 



Since the teratophthalmic head-form is determined by external 

 and physiological conditions which retard or inhibit head-develop- 

 ment to a greater or less degree, it is obviously a result of differen- 

 tial inhibition, the median region of the head being more suscepti- 

 ble and therefore more inhibited than the lateral regions. 



The teratomorphic head. This is merely a more extreme de- 

 gree of differential inhibition. This head-form usually shows a 

 single median eye (figs. 5 to 8) and a ganglionic mass retaining 

 only slight traces of the double structure of the normal ganglion 

 (Child and McKie, '11). Here the differential inhibition evi- 

 dently includes the median head region all the way to the tip, 

 for this region is more or less completely absent, and therefore 

 the cephalic lobes, lateral in normal heads, appear farther 

 anteriorly and more or less approximated to the median line, 

 until in the more extreme forms a single median cephalic lobe 

 with double (fig. 7) or even single unpigmented sensory area 

 at its base figure 8, develops. In these cases the whole 

 triangular region of the head between the cephalic lobes has 

 failed to develop. Figure 9 indicates diagrammatically the 

 regions chiefly inhibited in the various degrees of teratomorphic 

 development. 



There is some overlapping as regards the eyes between tera- 

 tophthalmic and teratomorphic forms. Occasionally in heads 

 distinctly teratomorphic as regards other features, more or less 

 distinctly double, instead of single eyes appear, and heads which 



