CLEAVAGE AND THE GERM LAYERS 



The Notochord or Chorda Dorsalis. Unlike in Amphioxus and 

 amphibia, the head (notochordal) process and mesoderm of higher verte- 

 brates are not clearly of entodermal origin, but are derived from the 

 ectoderm, any union with the entoderm being secondary. As the primi- 

 tive streak recedes caudalward during development the head process is 

 progressively lengthened at the expense of the former. Ultimately the 

 primitive streak becomes restricted to the tail region, whereas the entire 



Ectoderm 



Mesoderm 

 Entoderm 



Mesodermal segment 

 Ectoderm 

 Somatic 

 mesoderm 



Archenteron 



Entod, 



Archenter. 



Neural tube 



Nephrolome 

 Notochord 



Splanchnic 

 mesoderm 



Ccelom 



A B 



FIG. 29. Diagrams showing the origin of the germ layers of mammals as seen in transverse 

 section (modified from Bryce). 



remainder of the body is built up around the head process as an axis. In 

 later stages, the rod-like notochord extends in the midline beneath the 

 neural tube from the tail to a dorsal out-pocketing of the oral entoderm, 

 known as Seessel's pouch (p. 83). 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 amphibians. In man, traces of it are found 

 as pulpy masses (nuclei pulposi) in the intervertebral discs. 



