REPRODUCTION AND DEVELOPMENT 271 



ever, only the members of the class Amphibia pass through a free 

 swimming larval stage that feeds on plant materials in its environ- 

 ment. In all others development is direct toward the adult form; 

 nutrition is carried on in various ways that will be described 

 farther on. 



Fate of the Primary Layers. Development consists of chem- 

 ical differentiations, of cell divisions, and of growth of the various 

 primordia (p. 286) of adult structures, of foldings of layers and dis- 

 locations produced by unequal increase in relative size of the 

 developing structures, and finally of the perfection of the tissues so 

 that they may carry on their respective functions in the adult. In 

 normal development all these processes are accurately synchronized. 

 In vertebrates from the ectoderm are derived the outer layers of the 

 integument, integumentary glands, for example, sweat and other 

 skin glands, exoskeletal structures such as feathers in birds, the hair 

 and enamel of teeth in mammals, parts of scales in lower verte- 

 brates, and also all the nervous elements of the nervous system. The 

 ectoderm is also involved in the formation of two important endo- 

 crine glands, the pituitary and the adrenals (p. 204). From the 

 mesoderm are derived the nether layer of the integument, con- 

 nective tissues, the muscles, the excretory, reproductive, and circu- 

 latory systems, and supporting structures such as bone and cartilage. 

 From the endoderm are formed the lining of the digestive system, 

 digestive glands, parts of the urinary and genital systems, and the 

 lining of the trachea and lungs in air-breathing vertebrates. The 

 endoderm is also the origin of important endocrine glands, the thy- 

 roid, the parathyroids, and the pancreas (p. 205). 



The Neural Tube. The development of the nervous system 

 is accomplished by the greatest shifting of parts of layers in the 

 whole process of development, for it involves the penetration of 

 ectodermal derivatives deep into mesodermal and endodermal struc- 

 tures. In vertebrate embryos (Fig. 184) very soon after the begin- 

 ning of mesoderm formation the ectoderm anterior to the dorsal 



