xv j INTRODUCTION. 



shadows of their outlines. The difficulties of the last instance were usually 

 successfully met by the selection of a proper quality of gelatin plate and 

 by careful development and printing, which often brought out marked 

 contrasts. We occasionally resorted to the use of color screens with excel- 

 lent results, but in general they were not found necessary or of any par- 

 ticular advantage. The negatives, about 2,500 in number, were made by 

 Dr. Reichert, from which we have selected 600 to illustrate the text. 



The line-drawings, numbering 411, were made by Charles Travis, Ph.D., 

 instructor in geology and mineralogy in the University of Pennsylvania, 

 to whom we are especially indebted for the great care and accuracy with 

 which the work was done. 



While a large number of methods for preparing hemoglobin crystals in 

 large or small quantities have been published, it was found that in order to 

 obtain satisfactory results we should have to devise means whereby we could 

 have better control over the rapidity of crystallization, and also to avoid 

 any method which might injuriously affect the hemoglobin molecule. We 

 therefore devised methods for promoting or retarding crystallization. By 

 the former we have obtained crystals from small quantities of blood which 

 had not heretofore been obtained ; and by retarding crystallization we have 

 secured measurable crystals from blood in which, on account of their rapid 

 crystallizability, it has heretofore been impossible or difficult to develop 

 them. Moreover, by modifications of our processes we have in specimens 

 of blood of certain species been enabled to crystallize at will one or another 

 of several forms of oxyhemoglobin normally present in the same blood. 

 The pernicious effects of alcohol and of recrystallization led to the avoidance 

 of these agents. 



Finally, it was obvious that a successful outcome of our research 

 demanded an examination of specimens not only from a large number of 

 species, but also from species related and unrelated, so as to permit of a 

 critical examination of possible generic, family, and other peculiarities. 

 Such supplies as might be obtained from domesticated animals and such 

 small wild animals as could be secured within the possibilities of our grant 

 from the Carnegie Institution of Washington we realized could not meet 

 our necessities. We therefore sought the cooperation of those in authority 

 at the various zoological gardens of this country for specimens of blood 

 from animals that died. A circular letter was forwarded by President R. S. 

 Woodward to the management of each garden, and from a number of 

 them we obtained assistance. We are also indebted to Dr. S. Weir Mitchell 

 and to Dr. Charles D. Walcott for assistance in securing material. Speci- 

 mens were received from Mr. Stone, Rochester Park, N. Y.; Mr. M. P. 

 Hurlbut, commissioner of parks and boulevards, Detroit; Dr. H. H. Donald- 

 son, Wistar Institute of Anatomy; Mr. Ernest Tretow, Highland Park, 

 Pittsburg, Pa.; Mr. P. P. Randolph, Zoological Gardens, Seattle, Wash- 

 ington; Mr. R. G. Rau, Zoological Park, St. Joseph, Mo.; Dr. Herbert 

 Fox, pathologist of the Zoological Society of Philadelphia; Mr. Charles H. 

 Townsend, superintendent of the New York Aquarium ; Mr. H. A. Surface, 

 State zoologist of Pennsylvania; Dr. Frank Baker, National Zoological 



