REPRODUCTION AND DEVELOPMENT IN CHORDATES 



notochord (Fig. 5.15// and /). Somcwhal later the characteristic cavity ol 

 the mesoderm appears; this is the coelom (Fig. 5.17). With the localization 

 of the three germ layers and the formation of the archenteron and the coelom 

 as cavities surrounded, respectively, by the endoderm and the mesoderm, the 

 fundamental triploblastic body plan of the chordates has been established. 



The establishment of the typical l)ody plan is only the beginning of cell 

 localizations. Within each of the three layers movements of lesser masses 

 of cells occur and give rise to the primordia of the organ systems of the 

 adult vertebrate. 



At the same time that the mesoderm and notochord are being localized, the 

 first stage in the formation of the nervous system occurs. Fhe ectoderm over 

 the chordamesoderm becomes thicker and is known as the neural plate (Fig. 

 5.15//); when this occurs the embryo is sometimes called a neurula. Along 



Ectoderm 



Neural plate 



Dorsal mesoderm 



Notochord 



Endoderm 



Neural folds 



Notochord 



Neural groove 



Intermediate 

 mesoderm 



Neural 



Neural canal 



tube 



Coelom 



Neured crest 



Lateral 

 mesoderm 



Fig. 5.17. The localization of the neural tube in the domestic fowl. A, 

 B, and C, successive stages, in cross section. (Redrawn with modi- 

 fications from M. Duval, Atlas d'embryologie, 1889.) 



153 



