ORIGIN OF MONSTERS 525 



which he employed in removing fragments of tissue from the 

 antero-median and the antero-Iateral portion of the medullary- 

 plate has led to errors which Stockard apparently must have 

 overlooked. The unavoidable inaccuracy inherent in the method 

 of mechanically removing minute fragments is obvious. Even 

 with such refined methods as Spemann has employed and with 

 the experience and skill of the latter in such operations, this 

 inaccuracy can not be entirely avoided, as Spemann himself has 

 repeatedly pointed out. It must also be borne in mind that the 

 whole area from which the eyes can presumably arise, is in the 

 stage, at which Stockard performed his removal operations, rela- 

 tively very small. How could any satisfactory degree of pre- 

 cision be attained in operations performed on this small area 

 with 'fine scissors' (Stockard's method)? Is it not probable, 

 that while cutting out the antero-median region of the medullary 

 plate, Stockard has evidently removed also lateral material? 

 Or, that, while attempting to cut away antero-Iateral parts of 

 the medullary plate he evidently removed too little, being anxious 

 to avoid inaccuracies resulting from cuts carried too far from a 

 presumably correct position? In the medullary plate, the region 

 under discussion is — at least on superficial examination — mor- 

 phologically homogeneous, and the mapping out of morpho- 

 genetic areas for analytic experiments is beset with well-nigh 

 insurmountable difficulties, even if the instrument used for the 

 operation be the finest conceivable. This can readily be seen 

 from the results which Stockard reports to have obtained from 

 his experiments. Of nine embryos in which 'narrow strips' were 

 removed from the antero-median part of the medullary plate, 

 "four .... failed entirely to develop eyes." Of the five 

 other embryos only in one the eyes developed "to an extent 

 approaching the normal" and "four .... individuals pos- 

 sessed highly defective eyes" ('13, p. 288). 



Do such results warrant any conclusions at all? Ai'e not the 

 four cases, where defective eyes resulted from the removal of 

 the antero-median tissue, at least as conclusive for the lateral 

 position of two optic anlagen in the medullary plate as the four 

 eyeless embryos for the median position of a single optic anlage? 



