DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 389 
power of consuming degenerated tissues, and it has yet to be 
shown that the cells of the developing ovum are possessed of 
similar gastronomic powers. 
2. The Ectoderm at the Seventh Day. 
Both Selenka and Duval figure and describe the ectoderm at 
this stage as being formed by a number of distinctly outlined 
cells. My specimens, on the contrary, show clearly that the 
ectoderm is simply a nucleated protoplasmic mass. According 
to Duval the ectoderm surrounds the whole vesicle into which 
it projects at the proximal end as a plug-like mass, which is 
due to the rapid proliferation taking place in that situation. 
Selenka also describes an outer ectodermal wall on the sides 
and distal end of the vesicle which I have been eutirely un- 
able to find, and the cells figured by Duval as the representa- 
tives of this layer in the later stages are, in my opinion, modi- 
fied cells of the uterine mucosa. I base this opinion upon the 
fact that in my specimens the cells in question are not present in 
the early stages, for in the situations where they are figured by 
Selenka and Duval the hypoblast of the ovum is in contact with 
the uterine wall, and that they appear at a later period between 
the hypoblast and the uterine tissues simultaneously at irregular 
points, not as a layer extending gradually from the trophoblast. 
The ectoderm in ova of apparently the seventh day is 
described and figured by Selenka as consisting of two parts, 
the “‘ formative epiblast”? and the “Trager,” or, in other 
words, the epiblast and trophoblast. Selenka also draws 
attention to a cavity which appears in the trophoblast. 
Duval neither refers to nor figures the separation of the 
ectoderm into two layers, yet the separation is so evident 
(figs. 9, 10, 11, and 12, Pl. XXIII) that it is scarcely possible 
that he has not observed it ; and apparently he has not seen the 
cavity in the trophoblast (fig. 10, Pl. XXIII), possibly because 
it is present for a very short period, and he has missed the exact 
stage in which it occurs. This primary cavity in the tropho- 
blast is a special feature which must be due to some pecu- 
liarity of growth, but Selenka’s explanation that it is formed 
VOL, XXXII, PART Il].—NEW SER. DD 
