56 C. M. CHILD AND E. V. M. McKIE. 



disappearance of the median regions. Apparently \ve are jus- 

 tified in concluding that the reduction and di-.ippearance of 

 certain parts as the rate of reaction decreases is due one of two 

 alternatives: first, the reduced or absent part may represent a 

 relatively low rate of reaction in the normal animal and under 

 the experimental conditions the rate of the reaction which i> 

 essential for its formation approaches or falls below what m.iy 

 be called the morphbgenic threshold, i. e., it does not produce 

 the characteristic morphological effect. Second, a part may be 

 reduced or disappear under conditions which decrease the rate 

 of reaction, not because the reaction concerned in its formation is 

 directly affected by the experimental conditions, but because 

 its formation depends upon correlation with some other part 

 which is thus affected. It is probable, for example, that the 

 condition of the central nervous system in the abnormal forms 

 is largely, at least in the cephalic ganglia, a direct effect of the 

 experimental conditions, while the position, number and presence 

 or absence of the eyes and the degree of development of the 



* 



preocular region are to a considerable extent correlative effects. 



But however we may account for the results it is a demon- 

 strated fact that the reduction and disappearance of parts of so 

 "essential" an organ as the central nervous system can be 

 brought about experimentally by quantitative changes in ex- 

 ternal or internal conditions. No absence of chromosomes or 

 determinants and no germinal variation is necessary for the 

 production of these abnormal forms, but only a decrease in the 

 rate of the dynamic processes in the piece, together with the 

 necessary correlative effects of such a decrease. 



It is impossible to leave the subject without some reference 

 to the '"cyclopean" fish embryos which Stockard ('07, '09, '10) 

 has recently produced by means of magnesium chloride and 

 alcohol. The resemblance between these forms and the tera- 

 tophthalmic and teratomorphic forms of Planaria is striking. In 

 both cases organs which are normally bilaterally symmetrical 

 in position show various degrees of approach and in the extreme 

 types a single median organ develops in place of the two. An- 

 ophthalmic forms also occur in Planaria and under extreme ex- 

 perimental conditions completely headless forms also appe.ir. 



