THE ORIGIN OF THE MIDDLE GERM LAYER 47 



as taking its origin directly from the entoderm. Keibel points out that this con- 

 nection of the notochord is only secondary. The notochordal process grows 

 cephalad from the primitive node and the tissue from which it is derived is of 

 ectodermal origin, according to Keibel's view. In later stages, the notochord 

 extends in the midline beneath the neural tube from the tail to a dorsal out-pocket- 

 ing of the oval entoderm known as Seessel's pocket. It becomes enclosed in the 

 centra of the vertebrae and in the base of the cranium, and eventually degenerates. 

 In Amphioxus, it forms the only axial skeleton and it is persistent in the axial 

 skeleton of fishes and Amphibia. In man, traces of it are found as pulpy masses 

 in the intervertebral discs. 



