106 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. 

 In 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 



