ACKNOWLEDGMENTS 



To begin with, I cannot be too enthusiastic in expressing my 

 indebtedness to Dr. Carl Gottfried Hartman. His studies on 

 the opossum have been more extensive than those of any 

 previous investigator. That they have been liberally drawn 

 upon in the following pages will be apparent from the fre- 

 quent appearance of his name in the text. Not only has my 

 account of cleavage, germ-layer formation, and the rate of 

 development, been based primarily upon his researches, but 

 even my studies of the later stages which he did not examine 

 microscopically, have been made, for the most part, upon 

 material which he collected and left at The Wistar Institute. 



I have come to differ with Doctor Hartman 's interpretations 

 in several respects which are discussed in the appropriate 

 parts of the text ; but these differences, which, incidentally, do 

 not involve any differences in actual observations within the 

 stages he studied, came to light only after I had had the ad- 

 vantage of his fundamental researches and the opportunity 

 to study a large series of later stages of which he had no 

 sections, and which threw new light on the significance of the 

 earlier material. 



To Dr. C. H. Heuser, also formerly of The Wistar Institute, 

 I am indebted for the negatives of most of the photographs of 

 external views of the embryos, and for the use of figure 60. In 

 the few cases in which I had to make the negatives of external 

 views myself, the difficulties I encountered impressed me very 

 strongly with my indebtedness to his exceptional skill. 



To Dr. A. A. Zimmermann, of the Medical College of the 

 University of Illinois, I am indebted for the privilege of 

 reading in manuscript his account of the development of the 

 lymphatic system in the opossum, and for the use of two of his 

 photographs (figs. 48 and 59) in this book. 



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