350 G. CARL HUBER 
form and are continuous, in the mid sagittal plane, with cells 
that have wandered between the primary embryonic ectoderm 
and entoderm, cells regarded as constituting the mesoderm. 
Tn all of the sections of this series, so far as the mesoderm ex- 
tends, this is distinctly separable from the entoderm, and is 
continuous with the primary embryonic ectoderm only along a 
narrow region of thickened primary embryonic ectoderm, situ- 
ated in the mid-sagittal plane, and which may in this series be 
regarded as the anlage of the primitive streak. From the sides 
of this region of slightly thickened primary embryonic ectoderm, 
the extent of which is evidenced by the absence of an external 
limiting membrane, cells wander laterally to form the mesoderm. 
B, of figure 32, presents essentially the same appearance, although 
representing a slightly older stage. The sections of this series 
I regard as cut fairly well in a plane at right angles to the long 
axis of the respective egg-cylinder. The section taken for the 
sketch is situated a very little further away from the line of 
junction of the primary embryonic and extraembryonic ecto- 
derm, than is the section the drawing of which is shown in A 
of this figure, as may be Judged from the more uniformly pave- 
ment type of the entodermal cells. The triangular form of the 
proamniotic cavity is regarded as normal, and as indicating an 
early stage in the anlage of the primitive groove. In this figure, 
in its lower portion, the region of the primitive streak is readily 
discernible by reason of the fact that there is wanting here an 
external limiting membrane, and further by reason of the form 
of the cells and the form and relative position of their nuclei; 
certain of these cells indicating, both by their form and their 
position, the source and the direction of the wandering of the 
cells which constitute the anlage of the mesoderm. The wan- 
dering of the mesodermal cells between the primary embryonic 
ectoderm and entoderm, to form the lateral mesodermal wings, 
is clearly shown in this figure, especially to the left. The anti- 
mesometrial ends of the egg-cylinders, sections of which are 
shown in A and B of this figure, are as yet free from the invading 
mesoderm, as is also the part of the egg-cylinders lying opposite 
the region of the primitive streak, the upper portions of the 
