THE HISTOGENESIS OF THE ECTODERMAL DERIVATIVES 



303 



The intrinsic muscles of the larynx are differentiated from the mesenchyme 

 at the ventral ends of the third and fourth branchial arches. 



The muscles of the tongue are supplied by the n. hypoglossus and therefore 



Spinal ganglion 



Dermatome 



Ventral root 



Myotome 



Spinal nerve 



Arm bud 



Proliferating cells 

 of myotome 



ilesonephric duct 



Mesonephric tubule and 

 glomerulus 



Ccelom 



Somatic mesoderm 



Fig. 291. — Transverse section of a 10.3 mm. monkey embryo showing the myotome and the mesenchyma 

 of the arm bud (Kollmann's Handatlas). A, aorta; *, sclerotome. 



it has been assumed that they are derived from myotomes of the occipital region. 

 According to W. Lewis, "there is no evidence whatever for this statement, and 

 we are inclined to believe from our studies that the tongue musculature is derived 

 from the mesoderm of the floor of the mouth." 



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Pi* ■ .; 









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The Histogenesis of the Ectodermal Derivatives 



Besides forming the enamel of the teeth and salivary glands (see p. 161), the 

 ectoderm gives rise: (1) to the epidermis and its derivatives (subcutaneous 

 glands, nails, hair, and lens and cornea of the eye) ; (2) to the nervous system and 

 sensory epithelia ; (3) to parts of certain glands producing internal secretions such 

 as the pituitary body, adrenal glands, and chromaffin bodies. We shall describe 



