232 CHARLES K. STOCKARD 



the growing optic vesicles exert a depressing influence over the 

 growth of other parts. There is indirect experimental evidence 

 for such a statement. 



The initial moment of high cell multiplication for a particular 

 organ outgrowth is a most critical instant in the development of 

 this organ. Thus, if the general developmental rate of an em- 

 bryo be reduced by exposure to low temperature or cutting off 

 the oxygen supply at the time when the rapid proliferation of 

 the optic anlage should occur, the disproportionate growth of this 

 region is prevented, and the result of such an experiment will be 

 either the complete suppression of eye development, anophthal- 

 mia, Cyclopean eyes, monophthalamia, or some other degree of 

 defective eyes. This result ensues in spite of the fact that after 

 the critical moment for eye origin has passed the embryos may 

 have been again developing at the usual rate in a normal environ- 

 ment. The eye has only one favorable period for its origin, its 

 moment of supremacy so to speak, and when it is unable to ex- 

 press itself at this time, the opportunity is largely, if not entirely, 

 lost. This is probably due to other organ anlagen having arrived 

 at their controlling moments, the optic inhibition being no longer 

 sufficient or capable of suppressing them, but they, on the con- 

 trary, now suppress the optic bud. 



The arrest in development necessary for suppression of the 

 optic vesicle must be induced in the early embryo, before the 

 embryonic shield stage in the teleost, or before the optic anlage 

 is at all visible in the neural plate. This I ('09, '13) have shown 

 by a number of different experiments, and now also find to be 

 true in case of treatments with low temperature and scant oxy- 

 gen supply. 



I ('09) have reported a number of experimental cases of fish 

 embryos in which the eye was absent or was cyclopean, while the 

 general brain structures were as usual bilateral and normal. 

 Such specimens are viable and swim actively about. It is evi- 

 dent in these cases that the arrest was limited in its effect to the 

 optic outgrowths and was no longer effective when the primary 

 brain ventricles were forming. 



