STRUCTURE OF THE VERTEBRATES 



23 



Segmentation 

 cavity. 



Beginning of Gastrulation 



ctoderm 

 Endoderm 

 Archenteron 



Segmentation 

 cavity 



Blastopore 

 Completion of Gastrulation 



Fig. 5. Sagittal Sections of Amphioxus showing gastrulation. The ventral 



(endodermal) pole invaginates synchronously with the overgrowth of the 



dorsal (ectodermal) pole. 



gastrulation is complete is the blastopore which eventually gives 

 rise to the anus. 



Organogenesis, or organ formation, begins almost at once. The 

 development of the ectodermal dorsal nerve cord, the endodermal 

 notochord, and the mesodermal tissues are practically syn- 

 chronous. They are considered separately for convenience, and 

 constant reference should be made to the drawings. 



The nerve cord begins as a plate of cells (the neural or 

 medullary plate) along the dorsal, median line of the embryo. 

 The lateral margins of the plate soon break away from the 

 ectodermal cells covering the remainder of the embryo. The 

 ectoderm along either side of the plate becomes elevated, form- 

 ing neural folds, and these folds grow toward each other along 

 the dorsal midline. When the epidermal covering of the embryo 

 is about complete the neural plate sinks in the middle to form 

 a neural groove, and the continued infolding of the groove and 

 the upgrowth of the edges of the plate soon develop the dorsal 



