210 BULLETIN OF THE 



But even were it established beyond question that the organ is a 

 degenerate eye, there would still remain several quite distinct and very 

 interesting problems to be solved. The most fundamental of these is 

 pn)bably that of its homology. Much has been written on this ques- 

 tion by the various recent authors, but even less unanimity of opinioo 

 has been reached here than on the question of its structure and func- 

 tion. The question why the organ has remained so well developed in a 

 few systematically widely separated groups of vertebrates, while in all 

 others the process of degeneration has gone so far as to leave but a 

 mere trace of the proximal portion of the epiphysis, has not been 

 much discussed. It is not my purpose in the present paper to enter 

 upon a discussion of the theoretical questions involved, and they are 

 here adverted to merely to point out tlie need — as indicated by their 

 importance and the discordance of the opinions now held with regard 

 to them — of a larger body of facts on the subject than we yet possess. 

 For the present, I confine myself to a presentation of the facts observed, 

 and my interpretation of them as bearing npon some of the minor 

 conclusions reached by other writers, hoping to be able to pursue the 

 subject further in the near future, when situated in a region where au 

 abundance and a variety of material, adult and embryonic, can be ob- 

 tained, and where observations on the habits of the animals can be 

 made. 



The present work was undertaken at the suggestion of Prof. E. L. 

 Mark. I wish here to acknowledge my indebtedness to Mr. G. H. 

 Parker, of the Museum of Comparative Zoology ; to Mr. J. J. Rivers, 

 Curator of the Museum at the University of California ; and to Mr. T. 

 C. Palmer, of the United States Department of Agriculture, Wash- 

 ington, for material used ; and also to Mr. S. Garman, of this Museum, 

 for assistance in determining the species studied. 



A word as to technique. For studying the structure of the retina it 

 is very desirable to remove the great quantity of pigment that inva- 

 riably obscures the histological elements in this region. Neither nitric 

 nor hydrochloric acid, nor the alkalies, have any visible effect on this 

 pigment, but the desired result was reached by the use of chlorine gas. 

 The mounted, unstained sections were covered by a film of ninety per 

 cent alcohol, and placed in a tight glass chamber, in which was also 

 confined a small vessel containing a mixture of potassium chlorate and 

 hydrochloric acid for generating the gas. By being careful that the 

 slide on which the sections were mounted occupied a perfectly horizon- 

 tal position, and was so placed that the film of alcohol could not be 



