Fig. 68. The neural plate and the neural tube of salamander neurulae. Left: 

 The open neural plate with the neural ridges clearly formed. This is a head-on 

 view looking directly at the future brain. Right: The neural plate has folded 

 in such a way as to bring the neural ridges together in the midline of the embryo. 

 This is an oblique view, with the future brain at lower right and future spinal 

 cord fading into the upper left. The neural ridges are fusing in the center, but 

 the brain cavity and the cavity of the posterior neural tube are still open. (Cour- 

 tesy Professor John A. Moore, Columbia University.) 



Fig. 69. A surface view of an early gas- 

 trula showing the location of the presumptive 

 notochord, N: the presumptive mesoderm, 

 M, Afj ; and the presumptive endoderm, E, 

 E F) E v 



Fig. 70. A cross section through the pre- 

 sumptive mesoderm and presumptive noto- 

 chord after gastrulation. Note the beginning 

 of the separation of the mesoderm M, M t , 

 from the endoderm E, E x . A marks the 

 archenteron. 



