ORIGIN OF THE MESODERM, NOTOCHORD AND NEURAL TUBE 



between ectoderm and entoderm (Fig. 21 A and B). The size of the 

 invagination cavity varies in different species; in some it is elongated and 

 narrow, being confined to the middle line of the blastoderm. The floor of 



Ectoderm 



Blastopore 



Entoderm 



Notochordal 

 plate 



Remnant of floor 



FIG. 21. Longitudinal sections of the snake's blastoderm at various stages to show the origin 

 of the notochordal plate (adapted after Hertwig). 



this pouch soon fuses with the underlying entoderm and the two thin, 

 rupture, and disappear, thus putting the cavity of the pouch temporarily 

 in communication with the space (archenteron) beneath the entoderm 



Ectoderm 



Mesoderm 



Notochordal Entoderm 



plate 



FIG. 22. Transverse section of a snake's blastoderm at a level corresponding to the middle of 

 Fig. 21 C (adapted after Hertwig). 



(Fig. 21 C). The cells of the roof persist as the notochordal plate, which 

 later gives rise to the notochord. The neural folds arise before the mouth 

 of the pouch (blastopore) closes up, and, fusing to form the neural tube, 



