THE DEVELOPMENT OF THE INDIVIDUAL 531 



the mouth is formed, some of the ectoderm turns in for a short distance 

 and forms the enamel organs which produce the teeth. 



Meanwhile the primitive gut has divided into three regions : f oregut, 

 midgut, and hindgut. The foregut develops into the pharynx, esopha- 

 gus, and stomach ; the midgut into the small intestine ; and the hindgut 

 into the large intestine. The liver and pancreas are outgrowths from 

 the foregut and remain attached to it by their ducts. As the digestive 

 tube develops, it becomes attached to the inner layer of the mesoderm 

 and this forms the outer layers of the alimentary canal. Thus, the final 

 digestive tube is lined on the inside with endoderm and on the outside 

 by mesoderm. 



The Respiratory System. This system is primarily of endodermal 

 origin. In the frog, which respires by gills in the larval stage, a series 

 of endodermal evaginations from the pharynx meet a series of ecto- 

 dermal furrows, and the two sets fuse to form gill clefts. In land ani- 

 mals, these same pharyngeal evaginations occur in the embryo, but no 

 gills develop in connection with them. They are transient structures 

 which soon disappear, except the first one which sends off a pouch which 

 develops into the middle ear and Eustachian tube. 



Later in embryonic development a ventral outpocketing takes place 

 on the posterior floor of the pharynx. This pocket divides almost im- 

 mediately and forms the two lung buds. The original opening forms 

 the glottis which leads to the larynx. As the lung buds grow back into 

 the thorax, an elongated tube, the trachea, is formed in most chordates 

 and this branches to form the two bronchial tubes. In the frog the 

 trachea is lacking, and the bronchial tubes come off directly from the 

 larynx. As in the digestive system, the mesoderm covers the develop- 

 ing lungs and, thus, they are composed of two germ layers, endoderm 

 and mesoderm. 



The Skin and Its Outgrowths. The skin consists of two layers : the 

 epidermis which develops from the original ectodermal layer, and the 

 dermis which is mesodermal in origin. In the frog, a series of oval 

 glands develop from the epidermal layer which helps keep the skin 

 moist by their mucous secretions. In land vertebrates, the horny scales, 

 nails, claws, hoofs, feathers, and hair are ectodermal outgrowths of the 

 epidermis. The sweat glands and oil glands at the base of each hair 

 are also ectodermal ingrowths from the epidermis. 



The Circulatory System. Because the frog embryo is opaque the 

 development of the heart and blood vessels can be more easily demon- 

 strated in the thin, transparent embryo of the chick. This system, which 

 is of mesodermal origin, develops very early and is the first system to 

 become functional. Almost immediately it begins to move food to the 



