Med. Groove 
lMlesoblast 
AMesoblast 
Chorda ry “oblast 
Camera. 
Transverse section from the Head-Region of a Clemmys Embryo 3 days old. Zeiss DD >< 2. 
each side is also con- 
tinued into the meso- 
blast. Between the 
chorda-entoblast 
and darm-ento- 
blast, a diverti- 
culum of the ar- 
chenteron enters 
into. the ‚mesoz 
blastic mass on 
each side. The di- 
verticula are not pro- 
duced by disruption or 
other aceidents. They 
are, natural, ; the 
section is as clear as 
it can be, and it is 
impossible to make any 
mistake of observation 
on this point. It seems 
to me that this section 
can have only one in- 
terpretation. It re- 
presents, as cle- 
arly as we can 
reasonably expect 
in a meroblastic 
egg, the mode of 
the mesoblast- 
development so 
well known in 
Amphioxus. 
In Amphioxus, the 
mesoblast bounding the 
diverticula of the ar- 
chenteron is a single 
layer of cells. In the 
adjoined figure, on the 
internal side of the left 
diverticulum the meso- 
blast is in a single 
up Be 
