282 - Multicellular Animals, Especially Man 



NEURAL GROOVE- 

 NEURAL FOLD , 



Jlm$&y0$. 



MESODERM 



NEURAL TUBE^ 

 NOTOCHORD H 





NEURAL TUBE , 



NOTOCHORD J. _ 



ENDODERM 

 ECTODERM 



1 ENDODERM_J } ENDODERM J 



1 ECTODERM 1 ECTODERM 



B 



SOMITES 



SPINAL NERVE 

 SOMITES 



DIGESTIVE TRACT 

 COELOM 

 GENITAL RIDGE 



GILL CLEFTS 

 HEART 



Fig. 15-11. Some generalized stages in the development of a primitive verte- 

 brate. Stages A to E are cross sections cut from the mid-body region; whereas 

 F is a longitudinal section. 



nerves of the bod) represent outgrowths from 

 the neural tube. Each nerve consists of a 

 bundle of fibers that originate from nerve 

 cells as soon as these begin to differen- 

 tiate in the developing brain and spinal 

 cord. 



The flexible unsegmented notochord is a 

 skeletal rod that is found in all vertebrate 

 embryos, extending lengthwise through the 



embryo — between the neural tube and the 

 archenteron (Fig. 15-11 D,F). In vertebrates, 

 however, the notochord is a short-lived, ves- 

 tigial structure. In most cases it is totally 

 replaced by the segmented vertebral column; 

 but among primitive vertebrates (for exam- 

 ple, lampreys, sharks, and frogs), remnants of 

 the notochord can be found between the 

 vertebrae, even in the adult animals. 



