340 



REPRODUCTION 



Before long the entoderm spreads out over a larger area, 

 and from it and from the ectoderm is developed a layer of 

 cells, the mesoblast, or mesoderm, which occupies a position 

 between the other two layers. The three-layered germ is 

 now the blastodermic 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 forma- 

 tion of folds, ridges, constrictions, etc., and by various meta- 

 morphoses which have as their end the adaptation of struc- 

 ture to function. 



Derivatives of the Germ Layers. According to Heisler 

 these are : 



From the ectoderm: (i) The epidermis and its append- 

 ages, including the nails, the hair, the epithelium of the se- 

 baceous and sweat glands and the epithelium of the mam- 

 mary gland. (2) The infoldings 

 of the epidermis, including the 

 epithelium of the mouth and 

 salivary glands, of the nasal 

 tract and its communicating cav- 

 ities, of the external auditory 

 canal, of the anus and anterior 

 urethra, of the conjunctiva and 

 anterior part of the cornea, the 

 anterior 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 pos- 

 terior lobe of the pituitary body. 

 (4) The epithelium of the inter- 

 nal ear. 



From the entoderm: The epithelium of the respiratory 

 tract, of the digestive tract (from the back part of the phar- 

 ynx to the anus, including its associated glands, the liver and 



FIG. 97. Impregnated egg. 



With commencement of forma- 

 tion of embryo; showing the area 

 germinativa or embryonic spot, 

 the area pellucida, and the primi- 

 tive groove and streak. (Kirkes 

 after Dalton.) 



