CHAPTER ELEVEN 



Tlie Germ Layer Derivatives 



The Ectoderm and Its Derivatives Oculomotor 



The Brain Trochlear 



The Prosencephalon Trigeminal 



The Mesencephalon (Midbrain) Abducens 



The Rhombencephalon (Hind- Facial 



brain) Auditory 



The Spinal Cord Glossopharyngeal 



The Peripheral Nervous System ' Vagus 



The Organs of Special Sense The Spinal Nerves 



The Eye The Neural Crest Derivatives 



The Ear The Sympathetic Nervous System 



The Olfactory Organs The Adrenal Medulla 



The Lateral Line Organs The Stomodeum and Proctodeum 



The Cranial Nerves 

 Olfactory 

 Optic 



The Ectoderm and Its Derivatives 



The Brain 



The primary embryonic brain of the frog has three main sub- 

 divisions. The most anterior of these, the prosencephalon, alone be- 

 comes further subdivided into two regions, the telencephalon and the 

 diencephalon. In the higher vertebrates but not the frog, the rhom- 

 bencephalon (hindbrain) is also subdivided. The adult frog brain then 

 has four major divisions, arbitrarily determined, for they merge into 

 one another structurally and functionally. Each of these divisions 

 has a specific set of characteristics and derivatives, which will be de- 

 scribed in sequence from the most anterior to and including the spinal 

 cord. 



The Prosencephalon. 



The Telencephalon (Secondary Forebrain). The most ante- 

 rior division of the forebrain is the telencephalon with its original 

 cavity, the telocoel. One may draw a line in a sagittal plane from a 

 point just anterior to the epiphysis and extending through the brain 

 cavity to the oosterior wall of the thickened torus transversus. This 



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