BY JAS. P. HILL AXD C. J. MARTIN. 45 



General I)€SC7'iption of the Embryo as seen in Surface Vieio. 



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 1 9 mm. from the anterior end of the medullar}'- 

 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 embr3'o from the dorsal side magnified 

 54 diameters is shown in PL ix. Outside the elongated and 

 somewhat fiddle-shaped contour of the embryo is seen a lighter 

 more transparent zone (PL 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 l^oth 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 — (PL ix., jyra.) into which the mesoderm had not j^et 

 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 cerel)ral vesicles. The 

 outer contours of these head plates are jDosteriorly 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 b}^ the presence of a sulcus, the so-called head-fold, which 



