DERIVATIVES OF THE GERM LAYERS 



339 



mesoblast, or mesoderm, which occupies a position between the 

 other two layers. The three-layered germ is now the blasto- 

 dermic vesicle, or the gastrula, and its cavity is the archenteron, or 

 celenteron. From these three germ layers are developed all the 

 parts of the body by the formation of folds, ridges, constrictions, 

 etc., and by various metamorphoses which have as their end the 

 adaptation of structure to function. 



Derivatives of the Germ Layers. According to Heisler 

 these are: 



From the ectoderm: (i) The epidermis and its appendages, 

 including the nails, the hair, the epithelium of the sebaceous 

 and sweat glands and the epithelium of the mammary gland. 

 (2) The infoldings of the epidermis, including the epithelium of 

 the mouth and salivary glands, of 

 the nasal tract and its communi- 

 cating cavities, of the external audi- 

 tory canal, of the anus and anterior 

 urethra, of the conjunctiva and an- 

 terior part of the cornea, the ante- 

 rior lobe of the pituitary body, the 

 crystalline lens and the enamel of 

 the teeth. (3) The spinal cord and 

 brain with its outgrowths, including 

 the optic nerve, the retina and the 

 posterior lobe of the pituitary body. 

 (4) The epithelium of the internal 

 ear. 



From the entoderm: The epithe- 

 lium of the respiratory tract, of the 

 digestive tract (from the back part of the pharynx to the anus, 

 including its associated glands, the liver and pancreas), of the 

 middle ear and Eustachian tube, of the thymus and thyroid 

 bodies, of the bladder and first part of the male urethra and of 

 the entire female urethra. 



FIG. 97. Impregnated egg. 



With commencement of forma- 

 tion of embryo; showing the area 

 germinaiiva or embryonic spot, the 

 area pellucida, and the primitive 

 groove and streak. (Kirkes after 

 Dalton.) 



