Charles F. W. McClure . 165 
3. Three embryos of Didelphys averaging 11.5-12 mm. in length. 
These embryos were kindly presented to me by Dr. Bremer, of Harvard 
University, to whom my thanks are due. 
It is a curious fact that these embryos measure more than certain of 
the pouch young studied by the writer; a circumstance which shows, as 
suggested by Professor Minot, that opossums may vary considerably as 
to the degree of development attained before they enter the pouch. 
4. One pouch young of Didelphys measuring 10.5 mm. in length. 
Harvard Embryological Collection, No. 614. 
5. One pouch young of Didelphys measuring 11.5 mm. in length. 
Harvard Embryological Collection, No. 617. This measurement corre- 
sponds to that of Selenka’s, 87 (Fig. 2, Taf. XXIX), newly-born pouch 
young of eight days (eight days after beginning of cleavage), but is ap- 
parently not constant for young of this age, since the Harvard specimen, 
No. 614, which is undoubtedly the younger of the two, measures only 
10.5 mm. 
6. Five pouch young of Didelphys averaging about 14 mm. in length. 
%. Eight pouch young of Didelphys averaging about 15 mm. in 
length. According to measurements these correspond approximately to 
Selenka’s (87, Fig. 8, Taf. XXX) four days old pouch young (twelve 
days from the beginning of cleavage). 
8. One pouch young of Didelphys measuring 17 mm. in length. 
PREPARATION OF MATERIAL. 
The writer has experienced no difficulties as regards the fixation of the 
embryonic material. Any one of the ordinary fixing agents, such as 
picro-sublimate, Perenyi’s fluid, or a 10% solution of formaldehyde,’ 
will produce good results. The fixation of the pouch young, however, is 
a very difficult matter. In these the epitrichium is so impervious to the 
penetration of fluids that the writer has been unable to find any fixing 
2— take much pleasure in expressing my thanks and appreciations to the 
following gentlemen for the unusual courtesies they have extended to me in 
connection with the preparation of this paper: To Professor Charles S. Minot 
of Harvard University for the loan of the opossum material in the Harvard 
Embryological Collection; to Dr. Bremer of Harvard University for three 
opossum embryos and three pouch young; to Dr. J. P. Hill of the University 
of Sydney for an embryo of Dasyurus; to Professor Bashford Dean of Colum- 
bia University for a number of kangaroo pouch young; to Mr. Stephen S. 
Palmer of New York for funds necessary to cover the cost of several of the 
figures and plates in this paper, and to Professor Macloskie and Mr. Silvester 
of Princeton University for many helpful suggestions. 
*A 10 per cent solution of the 40 per cent commercial formaldehyde. 
