286 CHARLES R. STOCKARD 



fish are sometimes median and single, or occasionally bilateral 

 and more or less normal in position. I might also add that the 

 nasal pits are often absent. Again the 'defect hypothesis' must 

 expect median cells to fail and so allow the two nasal pits to 

 fuse medially. In this case the evidence is still stronger for the 

 primary median origin of the ectodermal anlage. Dohrn's stud- 

 ies on Ammococtes have shown the median position and relation- 

 ship of the nasal and hypophyseal invaginations. The question 

 of monorhiny in the cyclostomes is not fully determined but evi- 

 dence is certainly available to indicate a median nose anlage. 

 These, however, are phylogenetic considerations which would 

 only serve to prolong the present discussion. 



The experimental results presented by Spemann relative to 

 questions of lens formation agree almost entirely with the con- 

 clusions which were presented in my paper of 1910 b. He dis- 

 agrees, however, with many of my interpretations, yet I believe the 

 disagreement is not as complete as it often seems. 



I had suggested that the power of the ectoderm to form a lens 

 without the presence of an optic cup was less vigorous or efficient 

 than when the optic cup combined its stimulus with the tendency 

 to lens formation possessed by the ectoderm. For this reason 

 when the ectoderm was injured by many of the mechanical op- 

 erations which have been employed in the study of lens formation 

 the injured ectoderm was unable to form independent lenses al- 

 though normally it would have had such power. Attention was 

 called to the different results of Lewis and Miss King on Rana 

 palustris. 



Spemann ('12 a, p. 49) rejects this idea although he produces 

 evidence in his paper to prove its very probable correctness. 

 Compare the results he obtained in the origin of independent 

 lenses after cutting out the optic anlagen from medullary plates 

 with glass needles, in which case the ectoderm in the primary 

 lens-forming region was uninjured, with the results following a 

 burning out of the eye anlagen with hot needles, in which case 

 neighboring tissues were necessarily injured. After the latter 

 operation only one well formed lens occurred in five cases. A 



