GERM LAYERS. 



73 



itself. This transformation is initiated by an evagination dorsalward of the 

 entodermic cells which lie between the two mesodermic evaginations (Fig. 56, c), 

 these cells soon becoming constricted off as the solid cord of cells which consti- 

 tute the notochord (Fig. 56, d). With the separation of the chorda, the remain- 

 ing entoderm unites across the medial line and becomes the epithelium (en- 

 toderm) of the primitive intestine. The formation of the mesodermic somites 

 begins near the middle of the embryo and proceeds caudally. There is thus at 

 this stage a row of somites on each side of the medial line, the number of somites 



Notochord 



Entoderm 



Parietal 

 mesoderm 

 Visceral 

 mesoderm 



Intestine 

 Entoderm 



FIG. 56. From transverse sections through Amphioxus embryos, showing successive stages in for- 

 mation of mesoderm, neural tube and notochord. Bonnet. 



increasing by constant differentiation and pushing forward of more segments 

 (somites) from the caudal unsegmented mesoderm (Fig. 57). 



While the above described changes have been taking place, those ectodermic 

 cells which lie along the dorsal medial line become higher and form the bottom 

 of a shallow longitudinal groove. This is known as the neural groove, while the 

 folds which bound the groove on each side are known as the neural folds (Fig. 

 56, a). From the crests of the folds the remaining lower ectodermic cells grow 

 across and meet in the medial line thus forming the surface ectoderm (Fig. 56, 

 b and c). The neural groove next deepens, the neural folds bending dorsally 



