GASTRULATION IN VARIOUS CHORDATA 



413 



elevated and projected dorso-mediad. The exact limits of the neural plate thus 

 become evident (fig. 197D). 



3) Embryo Produced by the Gastrulative Processes. The general result of 

 epiboly and emboly in the Amphibia is the production of an embryo of three 

 germ layers with a rounded or oval shape. The potential skin ectoderm and 

 infolding, neural plate area form the external layer (fig. 192A). Underneath 

 this external layer are the following structural regions of the middle or meso- 

 dermal layer: 



(a) Below the developing nerve tube is the elongated band of notochordal 

 cells; 



(b) on either side of the notochord is the somitic (somite) mesoderm; 



(c) lateral to the somitic area is the mesoderm of the future kidney system; 

 and 



Fig. 195. Placement of the presumptive, organ-forrning areas in an embryo of Am- 

 phioxus of about six to seven somites. (Modified from Conklin, '32.) (A) Section 

 through anterior region. (J) Section through caudal end of embryo. (B-I) Successive 

 sections going posteriorly at different body levels between (A) and (J). 



