332 Charles R. Stockard 



imperfect formation or absence of one eye and entire absence of 

 eyes are all conditions common to the magnesium solutions and 

 very rare or never occurring in other solutions, nor in the hun- 

 dreds of eggs observed developing in sea-water. The conditions 

 are, therefore, probably due to a common cause, and I suggest 

 hypothetically that this cause is an inhibitory or anaesthetic effect 

 of the magnesium on the process of outpushing and separation 

 of the optic vesicles. Magnesium exerts a decidedly anaesthetic 

 effect upon both vertebrate and invertebrate animals and is an 

 inhibitor of muscular activity. It might possibly inhibit the giv- 

 ing off of the optic vesicles or prevent their separation in the brain, 

 so that both might come off together as in cyclopia, and it might 

 have caused the eye in Fig. 38 to be arrested when only halfway 

 separated from the brain; the absence of one eye and complete 

 absence of eyes would be perfect inhibition. It is necessary to 

 find a definite point in the strength of the solutions in order to 

 obtain the proper amount of inhibition for many weaker eggs are 

 killed during early stages. 



The strongest argument against such an hypothesis is the fact 

 that Mg in distilled water solutions fails to cause cyclopia, whereas 

 its anaesthetic or inhibiting powers should be most active in such 

 a solution. 



Dareste's ('91) idea that cyclopia is caused by a closed brain 

 or the failure of the anterior vesicle to develop is unsupported, 

 since in Triton with the hollow-brain tube present Spemann 

 finds that the defect occurs. In Fundulus the optic outpush- 

 ings are normally given off while the brain is yet solid, so that 

 according to Dareste all of these fish would be cyclopean in nature. 



Schwalbe ('06) in his Morphologie der Missbildungen des 

 Menchen und der Tiere, considers cyclopia to result from unusual 

 pressure exerted during early stages of development which does 

 not cause the lateral parts to grow together but prevents them 

 from developing at all. This position is somewhat in accord with 

 the hypothesis suggested above. If pressure prevents the grow- 

 ing apart laterally of the anlagen which normally require energy to 

 accomplish their separation, then by anaesthetizing a part, one 

 accomplishes practically the same thing as by applying pressure. 



