ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 



35 



The Notochord or Chorda Dorsalis.— Unlike in Amphioxus anci amphibia, the 

 head (notochordal) process and mesoderm of higher vertebrates are not clearly of 

 entodermal origin, but are derived from the ectoderm, any union with the entoderm 

 being secondary. As the primitive streak recedes caudalward during development 

 the head process is progressively lengthened at the expense of the former. Ulti- 

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

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



■Ectoderm 



■Mesoderm 

 Entoderm 



Archenteron 



Mes odermal seiment 

 Ectoderm- 

 Somatic 

 mesoderm 



Entoderm, 

 Archenteron. 



Neural tube 



Splanchnic 

 mesoderm 



Fig. 29. — Diagrams showing the origin of the germ layers of mammals as seen in transverse section 



(modified from Biyce). 



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 pocket 

 (p. 81). It becomes enclosed in the centra of the vertebras 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. 



