422 THE MAINTENANCE OF SPECIES 



Tissue Formation 



Each of the three primary germ hiyers produces a number of 

 different tissues that in turn form the various organ systems. Briefly 

 summarized, the ectoderm forms all of the nervous tissue, which in 

 turn makes up the nervous system, as well as the organs of special 

 sense that are developed in connection with it. The ectoderm also 

 gives rise to the epidermis of the integument and its various derivatives 

 such as scales, hair, horn, nails, feathers, and the enamel of the teeth. 

 In addition the linings of the mouth, anus, and nasal passages also 

 come from the ectodermal epithelial tissue. 



The endoderm forms the epithelial tissue lining the digestive tract 

 with the exception of its extremities which come from the ectoderm. 

 Many zoologists believe that all the various outgrowths from the 

 digestive tract, for example, the lungs, air tubes, and liver, as well 

 as various out-pocketings from the pharynx such as the thymus and 

 thyroid, contain a significant endodermal contribution. In some 

 chordates, the notochord buds oE from the endoderm. It should be 

 noted, however, that in the case of the lungs and liver considerable 

 amounts of mesodermal tissue also enter into the formation of these 

 organs. 



The mesoderm is the largest contributor to the tissues and different 

 systems of the body. The circulatory tissue is derived from the 

 mesenchyme of the mesoderm, while both skeletal and muscular 

 tissues and frequently the notochord come from this germ layer. 

 Likewise, both the excretory and reproductive systems are derived 

 from the mesoderm, which also makes some contribution to the 

 respiratory system. Finally the derma of the skin, cartilage, con- 

 nective tissues, such as ligaments and tendons, and the peritoneal 

 lining of the coelomic cavity, may be classified as mesodermal 

 derivatives. 



Protective Devices for the Embryo 



Egg Shells 



Various and sundry varieties of protective envelopes for ova are 

 found throughout the animal kingdom. Although protozoa do not 

 have eggs, encysted forms are protected from unfavorable environ- 

 mental conditions by hard coats analogous to shells. For example, 

 the cyst of Endameba histolytica, the causative organism of amebic 



