CHAPTER V 

 HISTOGENESIS— CLASSIFICATION 



Ectoderm, mesoderm, and entoderm (see page 63) are known 

 as the primary layers of the blastoderm. They differ from one an- 

 other not only in position, but also in the structural characteristics 

 of their cells. The separation of the blastomeres into these three 

 layers represents the first morphological differentiation of the cells 

 of the developing embryo. By further and constantly increasing 

 differentiation are developed from these three primary layers all 

 tissues and organs, each layer giving rise to its own special group of 

 tissues. The tissue derivations from the primary layers of the blasto- 

 derm are as follows: 



Ectoderm. — (i) Epithelium of skin and its appendages — hair, 

 nails, sweat, sebaceous and mammary glands, including smooth 

 muscle of sweat glands. 



(2) Epithelium of mouth and anus, of glands opening into mouth, 

 and enamel of teeth. 



(3) Epithelium of nose and of glands and cavities connected with 

 nose. 



(4) Epithelium of external auditory canal and of membranous 

 labyrinth. 



(5) Epithelium of anterior surface of cornea, of conjunctiva, of 

 crystalline lens, of lacrymal gland, and of lacrymal canal. 



(6) Epithelium of male urethra, except prostatic portion. 



(7) Epithelium of medulla of suprarenals, of pineal bodies, of 

 pituitary body and of a few other small ductless glands. 



(8) Entire nervous system, including retina and its forward 

 extension over iris, also muscle tissue of sphincter and dilator pupillae. 



Ei\jjpderm.- — (i) Epithelium of digestive tract (excepting mouth 

 and its glands and anus,) and of glands connected with digestive 

 tract including liver, gall bladder and pancreas. 



(2) Epithehum 01 respiratory tract and of its glands. 



(3) Epithelium of bladder except the trigonum, of female urethra, 

 of vaginal vestibule and glands of Bartholin, of prostatic portion of 

 male urethra, prostatic glands and glands of Cowper. 



69 



