6 Primitive Streak and Notochordal Canal in Chelonia. 



possesses an extraordinarily elongated streak, is seen to be a per- 

 fectly normal transverse slit, whose lateral horns are directed 

 anteriad and whose dorsal lip presents the median notch which 

 we shall later consider. The other embryo (^) lies anterior to 

 the first, and the axes of the two are almost perpendicular to 

 each other. At the hinder end of the second embryo (^) occurs 

 a crescentic depression. 



Diagram I. 



The axes of these twins were so nearly at right angles, that it 

 was possible, without changing the plane of sectioning, to get an 

 approximately sagittal section of one embryo and a transverse 

 section of the other. For the sake of ease in description, the 

 embryo which possesses the elongated streak will be designated 

 a, and the one which lies anterior and at right angles to a will be 

 named /3. The embryos were so oriented that a was cut into 

 transverse and /S into sagittal sections. 



The forty-seventh section (Plate IV. Fig. 14), counting from 

 the right edge of ^, is median, but makes a small angle with 

 the sagittal plane of /3. This section discloses the fact that the 

 invaginated cavity possesses " two limbs, one vertical and one hori- 

 zontal," which makes it correspond with one of Mitsukuri's ('93, 



