372 



THE NEW INDIVIDUAL 



Part IV 



Future skin 



e nerve 

 ord 



Blastopore 

 of cells 



Future yolk plug 

 A. LATE BLASTULA 



B. GASTRULA STAGE 



Future skin 



Future nerve 

 cord 



Arch 



enteron 



Future brain 



Archenteron 



Notochord 



Future nerve 

 cord 



Notochord 



Yolk plug 



Yolk plug 



C. GASTRULA 



D. LATE GASTRULA STAGE 



Fig. 19.9. Internal views of frog embryos. (From Development of the Frog, as 

 illustrated by the Mueller-Ward Models. Courtesy, Justus F. Mueller and Ward's 

 Natural Science Establishment.) 



Skin and Nervous System. After the neural groove is closed there is a short 

 passageway between the neural tube and enteron, the neurenteric canal, that 

 exists but a short time (Fig. 19.10). The forebrain, midbrain, and hindbrain 

 gradually take shape. Beneath and near the front of the forebrain a process of 

 superficial ectoderm extends inward. This later joins a downpushing of the 

 brain and together they become the pituitary body. From the ventral side of 

 the neural tube motor nerve cells send out processes to muscles and glands. 

 Processes from the cells of the dorsal ganglia extend into the cord, to the skin 

 and to other parts of the body. The cord and brain are gradually surrounded 

 by an envelope of loose mesodermal (mesenchymal) cells. In all vertebrates 

 such cells form the coverings or meninges of the spinal cord. 



The lining of the neural tube is a center of active cell division and gradually 

 increasing differentiation. Two types of cells are formed, the future supporting 

 cells or neuroglia of the nervous system and the nerve cells or neurons. The 

 latter move out of the lining into the thick wall of the neural tube where they 

 develop into typical neurons with extended axons and dendrites (Fig. 16.3). 



