THE NOTOCHORDAL CANAL. 53 



This thickening is termed the primitive axis. German writers commonly designate 

 it as the "head process" (Kopffortsatz) . The primitive axis may be easily distin- 

 guished in transverse sections from the primitive streak by the fact that in the for- 

 mer the thickening occurs in the mesoderm and entoderm, which are closely united, 

 and it is separated from the outer layer; whereas in the latter the cells of the 

 thickening are fused with both the entoderm and the ectoderm (compare Fig. 126, 

 A and C, page 171). 



The primitive axis corresponds to the region in which the body proper of the 

 embryo develops, and represents the beginning of embryonic development in this 

 restricted sense. It grows quite rapidly in length and width, and as it grows en- 

 croaches more and more upon the territory of the primitive streak, which is grad- 

 ually obliterated by merging into the caudal end of the developing embryo, so that 

 it can no longer be distinguished. The obliteration of the primitive streak is grad- 

 ual, and there is a series of stages easily observed in amniota in -which we find the 

 embryonic development in the region of the primitive axis more or less advanced, 

 while part of the primitive streak still presents to us, more or less clearly, its 

 original condition. 



The Notochordal Canal. 



In regard to this canal our knowledge is imperfect. Any account of it which 

 we can give may need correction. It is a very small canal which runs through the 

 center of the primitive axis. It ends blindly in front, but opens through the ecto- 

 derm at its posterior end, at a point corresponding perhaps exactly to the position 

 of the primitive knot. The first indication of the formation of the canal is an al- 

 teration in the form of the cells in the center of the primitive axis. These cells 

 elongate in directions at right angles to the axis. Their nuclei become oval and are 

 radially placed. The change' begins posteriorly and progresses forward. The radial 

 cells move apart, so that there arises a longitudinal canal. It may happen that in 

 mammals, as in birds, the canal is not actually open at its posterior end. If that 

 should be found to be the case in any instance, it would not alter our interpreta- 

 tion, for we should then consider that the walls had simply closed togethe r. There 

 are many instances of tubular structures being temporarily solid in embryonic stages. 

 Such a condition, for example, has been observed in the oesophagus of elasmo-^ 

 branchs, in the large intestine of birds, and in other cases. 



The opening of the notochordal canal is termed the blastopore, and is suppose'd 

 to be identical with the blastopore of the anamniota. 



After the notochordal canal is formed the blastodermic vesicle has, of course, 

 two cavities: first, the small cavity of the canal; second, the large main cavity of 

 the vesicle which is surrounded by entoderm. This larger space is designated as 

 the yolk-cavity. After the canal has acquired a not inconsiderable length its lower 

 wall develops a series of irregular openings (Fig. 20, nch] on its ventral side, by 

 which it comes into communication with the large underlying yolk-cavity. These 



