476 WM. E, KELLICOTT 



support Child's 'axial gradient' theory in explanation of the 

 frequency with which there occur defects of the tissues and 

 organs developing in the anterior end of the embryo. ''When 

 the egg is acted upon by a toxic substance, a restricted area at 

 the anterior end of the embryo's median body axis becomes so 

 altered chemically as to be eliminated from further development 

 or it may go on developing to a certain point beyond which it is 

 chemically unable to proceed" (p. 557). "The size of the in- 

 jured area at the anterior end is probably subject to considerable 

 variation," and thus the effects may be hmited to the future 

 interocular area, or they may include parts, varying in extent, 

 of the potential optic anlagen, or one optic anlage only, and so 

 on. In addition to these assumptions, it is further necessary to 

 assume (p. 558) not only the existence of differentiated ophthal- 

 moblastic anlagen in the very early cleavage group, but definite 

 and symmetrically placed double ophthalmoblastic anlagen with 

 different degrees of susceptibiUty to the chemical substances in 

 solution. By means of these and other assumptions (p. 558) 

 Werber is able to reconcile the two hypotheses of the causes of 

 cyclopia mentioned above, but in accomplishing this he runs 

 contrary to the demonstrated lack of specification or deter- 

 mination in the cleavage group of the Teleost. Eggs were 

 treated in the one- to sixteen-cell stages, and as Werber himself 

 remarks (pp. 531-2) it is probable "that it is mainly the initial 

 effect of the toxic solution on the ovum that causes it to develop 

 in an atypical manner." There is no anterior end of an embryo 

 represented by any differentiated material in the sixteen-cell 

 stage of the Teleost, no ophthalmoblastic anlage; but there is an 

 organization or developmental mechanism capable of producing 

 these parts, much later, a mechanism that is interfered with 

 and upset, and there is no specificity in the result of the de- 

 rangement. This lack of specificity is directly opposed to the 

 application of the axial gradient hypothesis, for as a matter of 

 fact, any part, posterior as well as anterior, may become abnor- 

 mal following this or other modes of treatment. 



The explanations for the observation that all of the organs of 

 the anterior end of the Teleost embryo — eyes, brain, heart, etc., 



