DEVELOPMENT OF THE OPOSSUM 3 
stages, were also collected during 1916, and at this time many 
more eggs and embryos were sacrificed for a series of physio- 
logical experiments on the female opossum. As a result of 
these experiments I learned a simple and comparatively certain 
means of diagnosing a’ female opossum in the earliest stages of 
pregnancy and in early oestrus. Since it was felt that this ex- 
perience would greatly facilitate collecting in 1917, plans were 
made to secure a complete series of eggs, embryos, and pouch 
young of this species. ‘The more than hoped for success of the 
effort was due to the active interest of Dr. M. J. Greenman, 
Director of The Wistar Institute, for it was through the generosity 
of the Institute that I was enabled to secure and care for the requi- 
site number of animals and also to have the advantage of the able 
services of Dr. C. H. Heuser, embryologist at the Institute, who, 
with the assistant of Miss Aimée Vanneman, technician in the 
School of Zoology, the University of Texas, saw to the proper 
fixation and after-treatment of the specimens. Entire credit 
also belongs to Doctor Heuser for the unique series of photo- 
graphs of living eggs, some of which are herein reproduced. To 
Dr. J. T. Patterson is due the initiation of the work on this 
interesting marsupial, and his scientific zeal and keen interest in 
mammalian embryology have been a constant inspiration to the 
writer. I am indebted for indispensable assistance in the 
operations on the animals during the last two years’ collecting to 
a number of premedical students of zoology, notably to Mr. 
Victor Tucker, who stood ready to help at any hour of the day 
or night and who performed many of the operations; also to 
Miss Janoch and Messrs. Goff, Stiefel, and Kaliski. 
During the year 1917-18 I have enjoyed the privileges of a 
fellowship at The Wistar Institute and have been its guest while 
engaged in a study of the material collected. I am further 
indebted through the Institute to Dr. C. H. Heuser for making 
some of my best preparations of serial sections and to Mr. T. 
H. Bleakney, artist at the Institute, for drawing plate 12 and for 
shading and finishing the figures drawn for this paper. 
The new material collected since the publication of my former 
paper covers many stages there described, and in addition 
