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GENERAL CONCEPTS 



However the mesoderm may originate, it typically forms two sheets 

 which grow laterally and anteriorly between the ectoderm and endo- 

 derm; one sheet becomes attached to the inner endoderm and the other 

 to the outer ectoderm. The cavity between the two becomes the coelom, 

 or body cavity. The layer ot mesoderm associated with the endoderm 

 forms the muscles of the digestive tract. 



The primitive skeleton of the chordates is the notochord, a flexible, 

 unsegmented, longitudinal rod which occurs in the dorsal midline of 

 all chordate embryos. It is formed at the same time and in a similar 

 way as the mesoderm— as an outgrowth of the roof of the archenteron, 

 from the dorsal lip of the blastopore, or from the primitive streak. Later 

 in the development of vertebrates the notochord is replaced by the ver- 

 tebral column, derived from part of the mesoderm. 



The nervous system of chordates is derived from the ectoderm over- 

 lying the notochord. This first forms a thickened plate of cells, the neural 

 plate; the center of the plate becomes depressed while the lateral edges 

 rise as two longitudinal neural folds. The folds eventually meet dorsally 

 and form a hollow neural tube. The cavity of the tube becomes the cen- 

 tral canal of the spinal cord and the ventricles of the brain. 



The sheets of mesoderm grow ventrally and the ones from either 

 side meet in the ventral midline; the coelomic cavities on the two sides 

 then fuse into one. The mesoderm grows dorsally along each side of the 

 notochord and neural tube and becomes differentiated into segmental 

 blocks of tissue, the somites, from which the main muscles of the trunk 

 develop. Other mesodermal cells become detached from the inner border 

 of the somites, migrate inward, surround the notochord and neural tube, 

 and develop into the vertebrae. The kidneys and their ducts, and the 

 gonads and their ducts, are derived from the mesoderm originally located 

 between the somites and the coelom. 



The contributions of each germ layer to the development of a typical 

 mammal are summarized in the following table. 



ECTODERM 



Epidermis of the skin 

 Hair and nails 

 Sweat glands 



Brain, spinal cord, ganglia, 



nerves 

 Receptor cells of sense 



organs 

 Lens of the eye 



Lining of mouth, nostrils 



and anus 

 Enamel of teeth 



ENDODERM 



Lining of gut 



Lining of trachea, bronchi 



and lungs 

 Liver 



Pancreas 



Lining of gallbladder 



Thyroid, parathyroid and 



thymus glands 

 Urinary bladder 



Lining of urethra 



MESODERM 



Muscles — ^smooth, skeletal 



and cardiac 

 Dermis of skin 



Connective tissue, bone 



and cartilage 

 Dentin of teeth 



Blood and blood vessels 



Mesenteries 



Kidneys 



Testes and ovaries 



The details of vertebrate development will be given in Chapter 31. 



