416 C. M. CHILD 



and in the form of the head. Obviously, the normal head rep- 

 resents a certain proportionality of parts which in some way is 

 determined as the usual result of development. 



The teratophthalmic head. The shape of the head is normal, 

 but the eyes show more or less approximation to the median 

 line, and section shows that the gangha are also more or less 

 approximated with reduction or complete absence of the com- 

 missure between them (fig. 2; also Child and McKie,'ll). Figure 

 3 shows some of the eye-forms characteristic of the teratoph- 

 thalmic head, the first five horizontal rows of the figure repre- 

 senting bilaterally symmetrical, the last two rows asym- 

 metrical forms. The first four rows show forms in which more 

 or less continuity of pigment appears, even when the eyes are a 

 considerable distance apart. In the fifth row are shown cases in 

 which the pigment cups remain distinct until the eyes are rather 

 closely approximated. The order in each series follows in gen- 

 eral the degree of departure from the normal eye-form. The 

 asymmetry of the series in the last two rows of figure 3 is the 

 result of purely incidental factors which determine a difference 

 in rate of development on the two sides of the median line. For 

 example, obliquity of cut surface may determine temporary 

 asymmetry, the more posterior levels developing more slowly be 

 cause they represent lower levels of the longitudinal gradient. 

 Again, if the amount of entodermal tissue exposed at the cut 

 surface is much greater on one side than on the other, the expo- 

 sure of parenchyma is less and the rate of head-development is 

 slower on the side with the greater entodermal exposure. The 

 asymmetries of the eyes in this series are merely indications of 

 the ganglionic asymmetries. In many cases both disappear in 

 later development. 



It is evident that the teratophthalmic head differs from the 

 normal in the reduction or complete inhibition of the median 

 region of the head, at least at the level of the ganglia. The degree 

 of inhibition may range from the just appreciable approximation 

 of eyes and ganglia to the complete inhibition of development 

 of the region lying between the dotted lines in figure 4. Whether 

 the inhibition of the median region extends to the tip of the head 



