82 CARL G. HARTMAN 
b. The bilaminar blastocyst according to Selenka 
In his ‘Studien’ (’87) Selenka briefly describes two opossum 
blastocysts of about 1.1 mm. in diameter. The lithographs pre- 
sented by him are idealized drawings, reconstructed from his 
sections, which I judge to have been considerably shrunken by ~ 
the treatment to which they were subjected. The illustrations 
give the correct relation of the structures except for the diffuse 
junctional line, although Selenka is in error as to the homology 
of the ‘Granulosa membran,’ as he terms the shell membrane. 
c. The 1-mm. blastocyst according to Minot 
In 1911 the late Professor Minot published a description of 
six 1-mm. blastocysts of the opossum, of which two were fixed 
in Flemming’s fluid and four in Zenker’s and of the latter, two 
were fixed in situ with the uterus. 
He gives an adequate description of the embryonic area of 
this stage, as did Selenka in 1887. Of especial interest, however, 
is Minot’s description and interpretation of certain cells in the 
trophoblastic area. In a vesicle which he dissected and mounted 
flat on a slide he found numerous large light areas apparent as 
‘minute round holes’ when viewed with a hand lens. He inter- 
preted these areas as gaps in the ectoderm filled with entodermal 
cells which thus reach the surface at these points. Corrobora- 
tion was found in the study of the serial sections. The author 
furthermore draws a comparison between these large lightly 
staining entodermal cells, which rise to the surface in the tropho- 
blastic region of the opossum egg, and the small darkly staining 
entoderm mother cells which appear in the embryonic ectoderm 
of the Dasyurus blastocyst. 
While the preparation of the present paper was in progress I 
had the privilege of studying Professor Minot’s specimens at 
the Harvard Medical School. As I expected, the serial sections 
of eggs fixed and sectioned in toto with the uterus are badly 
shrunken and filled with coagulum in a manner which never 
occurs in eggs removed from the uterus and treated separately. 
