BY JAS. P. HILL AND C. J. MARTIN. 
45 
General Description of the Embryo as seen in Surface Vieiv. 
On opening the egg, the embryo was found lying on the surface 
of a thin-walled vesicle, with its long axis corresponding to the 
long diameter of the egg. It extended over the surface of the 
vesicle almost from pole to pole. The vesicle completely filled 
the interior of the shell. It contained a thin whitish transparent 
fluid of an albuminous nature which was precipitated in picro- 
sulphuric acid. Immediately below the wall of the vesicle there 
appeared a thin layer of yolk granules which was somewhat 
increased over a small area at the ant-embryonic pole. The 
embryo measured 19 mm. from the anterior end of the medullary 
plate to the extreme posterior end of the primitive streak. This 
hinder point of measurement is 1*5 mm. behind the blastopore. 
A photo-micrograph of the embryo from the dorsal side magnified 
5|- diameters is shown in PL ix. Outside the elongated and 
somewhat fiddle-shaped contour of the embryo is seen a lighter 
more transparent zone (PI. ix. am. a.) corresponding to the amniotic 
area of other mammals. In the fresh condition no trace of a 
vascular area was visible, though in the hardened blastoderm 
developing vessels were indicated by a mottling both in and 
around the amniotic area. Immediately in front of the anterior 
end of the embryo there was to be seen a lighter area — the 
proamnion — (PI. ix., pra.) into which the mesoderm had not yet 
extended. The antero-lateral portions of the embryo were almost 
entirely occupied by two sharply limited patches situated one on 
either side of the anterior region of the medullary plate, opposite 
the position of the future first and second cerebral vesicles. The 
outer contours of these head plates are posteriorly in line with 
the forward continuations of the outer borders of the proto- 
vertebral zones of mesoderm. The outer margins of these head 
plates mark the lateral limits of a very considerable mesodermal 
thickening in this region, and we may for convenience of descrip- 
tion term them the head plates of mesoderm. Their relations to 
the general mesoderm will be described later. 
The commencing separation of the embryo from the vesicle is 
indicated by the presence of a sulcus, the so-called head-fold, which 
