POSITION OF OPTIC ANLAGE IN AMBLYSTOMA 277 



absence of any other brain part is in entire accord with the 

 above facts. 



It is believed, therefore, that the various degrees of the typi- 

 cal Cyclopean condition from eyes unusually close together, to 

 median double or hour-glass eyes, to the large oval eye, to a 

 median round eye of usual size, to the median eye smaller than 

 one normal eye, and finally to complete failure of eye material 

 to arise from the brain are probably all due to developmental 

 arrest. The arrest in development is the result of some influence 

 which has reduced the developmental vigor below the normal 

 so that the energy is not available to carry out the usual pro- 

 cesses of differentiation and growth. 



The author has figures and described other cases of eye de- 

 fects which do not fall exactly into the series of cyclopia as 

 considered above; yet these cases are modified conditions which 

 are closely related to the Cyclopean series both in point of origin 

 and in their final condition. The curtain-like eyes which face 

 the median plane and often have a single lens between them 

 might be considered as delayed cases of cyclopia. The eye an- 

 lage widened, the eyes became separated and continued their 

 diff"erentiation, yet they were unable to turn out and assume 

 their normal lateral positions so that they faced in a ventro- 

 median dkection with their anterior walls closely approximated 

 as is shown in figure 38 ('09) and figures 1, 4, 5, 6, 13, 14 ('10 a). 

 Such eyes often excite a single stimulus upon a more or less ven- 

 tral ectodermal region which responds by forming a single lens 

 lying between the two eyes. 



The writer has illustrated by a diagram (fig. 15, A and B 

 '10 a,) the difference between these eyes with their choroid sur- 

 faces against the lateral ectoderm of the head and their concave 

 retinal surfaces facing medially, and the eyes of a normal indi- 

 vidual ('10 a p. 380). As was then stated, the experiments did 

 not give a definite clue to indicate the position of the optic 

 anlagen in the early brain, thus my explanation, or 'laming hy- 

 pothesis,' referred mainly to the pushing out and lateral develop- 

 ment of the optic vesicle and cup. All these cases are decidedly 



