526 E. I. WERBER 



Is it not safe to hold that in these experiments the results can 

 not be read aright, owing to the great probability of an error 

 introduced by the inadequate method of the experiment? It 

 would seem obvious that, if experiments be performed on the 

 said area at this stage of development with an instrument as 

 crude as the one employed by Stockard, the deductions from the 

 results are bound to be either erroneous or at least very unsafe 

 in almost every case. 



For the validity of this claim of the 'single eye anlage' and its 

 'median position in the medullary plate' Stockard ('13, pp. 274- 

 276) attempts another proof. He holds that the position of the 

 optic cross outside and below the brain would be inconceivable, 

 if the optic anlagen should be lateral in position and other brain 

 tissue be present between them. For, then, he concludes, the 

 optic stalks instead of having a ''median origin and connection" 

 would be "attached to lateral regions of the brain from which 

 the optic vesicles pushed out." According to his diagram pre- 

 sented in figure 8 (p. 276) ". . . . in the course of develop- 

 ment the fibers of the optic nerve following the stalk reach the 

 lateral position and must enter the brain and continue within 

 its tissue in order to meet the nerve of the opposite side and 

 form the cross or chiasma. Brain tissue would lie beneath the 

 optic chiasma" and "this condition is never found in any normal 

 vertebrate." 



These deductions would have to be regarded as very important 

 if they were correct. But, as will be seen from the following, 

 they will not hold good. 



At the outset it should be said that the optic vesicles do not 

 'push out.' Instead they are being pushed out by the pri- 

 moridum of the brain. The parts of the brain anlage (and not 

 of the eye vesicles, as Stockard suggests) most directly concerned 

 in this process of pushing out the anlage for the eye, elongate 

 more and more, until they have attained the form of optic stalks 

 at the time when the differentiating eye vesicles have reached 

 their final positions in the head. Their origin is just as much 

 from tissue dorso-lateral as from ventro-median to the optic 

 vesicle. It is true that this view is, as yet, not based on experi- 



