THE EMBRYO 129 



are composed of ME and M^E^ respectively. At the end of gastrulation the 

 lateral mesoderm, M and M lf separates from the endoderm, E and E x , and 

 begins to migrate ventrally, as shown by the arrows. E and E x migrate dor- 

 sally, and thus the two groups of cells move in opposite directions. 



The migration of endoderm continues, as in Figure 71, left, and is com- 

 pleted in Figure 71, right, where the roof of the archenteron is composed 

 entirely of endoderm. The migration of the lateral mesoderm is indicated by 

 arrows leading from M and M x . The two masses of lateral mesoderm finally 

 join in the midventral line and split to form the splanchnic (inner) and 

 somatic (outer) layer of mesoderm. The subsequent fate of the mesoderm 

 will be discussed in Chapter 14. 



Formation of the neural tube 



While the above processes are taking place, the primitive nervous system 

 begins to differentiate from the ectodermal layer of cells. First a plate of cells 

 above the roof of the archenteron forms the neural plate (Fig. 71). The 

 edges of the plate form the neural ridges, C, C ± . The neural plate folds in 

 such a way as to form a neural tube with the neural ridges coming together 

 and fusing in the midline of the embryo. Figure 68 shows the neural plate 

 and the neural tube of a salamander. 



More details concerning the development of the nervous system will be 

 considered in Chapter 11. At this time we shall simply point out the spatial 

 relationship between the ectoderm, which forms the neural plate, and the 

 notochord and mesoderm, which come from the region around the dorsal 

 lip of the blastopore in the very early gastrula. The mesoderm and notochord 

 are in firm contact with the future neural plate during gastrulation. 



The embryo 



After the closure of the neural folds, the formation of the mesoderm, 

 and the early differentiation of the notochord, the neurula becomes elongated 

 and the head begins to form. These form changes are illustrated for the frog 

 by Figure 67, stage 17, and for the salamander by photographs of a neurula 

 and early embryo, Figure 72. At about stage 17 the neurula becomes an 

 embryo, although there are no hard-and-fast lines between these two stages 

 of development. The early embryo is characterized by an increase in recog- 



