To Dr. C. F. W. McClure, of Princeton University, for the 

 loan of several series of sectioned pouch young, and for figure 

 50; to Dr. J. L. Bremer, of the Harvard Medical School, for 

 figure 58 ; to Dr. J. Duesberg, of the Faculty of Medicine, Uni- 

 versity of Liege, Belgium, for permission to copy some of his 

 drawings for my figure 3; to Dr. Th. S. Painter, of the Uni- 

 versity of Texas, for figure 2 ; and to Dr. R. C. Hutchinson, of 

 The Wistar Institute, for the photograph of my wax model in 

 figure 11 ; I express my very hearty thanks. 



And finally, my greatest debt of gratitude is, of course, to 

 the late Dr. M. J. Greenman, Director of The Wistar Institute, 

 whose unflagging interest in the opossum over a number of 

 years made possible not only my own work, but that of Doctor 

 Hartman, and Doctor Heuser, and Doctor Zimmermann, as 

 well. 



10 



