DEVELOPMENT OF THE INDIVIDUAL 



567 



from which a complex metazoan body can 

 be built. The layers grow, fold, and differen- 

 tiate into the adult structures. In the Meta- 

 zoa in general, the endoderm becomes the 

 epithelial lining of most of the digestive 

 tract and its glands. The ectoderm gives 

 rise to the outer layer of the skin, the nerv- 

 ous system, and the sensory cells of the 

 adult. The two layers of the mesoderm form 

 different parts: the inner portion becomes 

 associated with the endoderm to form the 

 gut wall, while the outer becomes associated 

 with the ectoderm to give rise to the body 

 wall, thus producing the familiar "tube- 

 within-a-tube" body plan discussed in ear- 

 lier chapters. Between the two tubes is the 

 cavity of the mesoderm, the coelom. Deriv- 

 atives of these layers in man are graphically 

 portrayed in Fig. 23-5. 



Organ formation 



It is a long, complicated story from the 

 simple germ layers to the full-fledged func- 

 tional organs which, taken together, consti- 

 tute the complete animal. This is accom- 

 plished by several methods. One of the most 

 common is the folding of the layers, which 

 is well illustrated in the formation of the 

 nervous system of Amphioxus and of the 

 chordates in general (Fig. 23-6). It starts 

 by a pushing in of a groove along tlie dorsal 

 side of the embryo about the time the meso- 

 derm is established. The groove closes over 

 on the dorsal side and sinks below the sur- 

 face to form a tube which gives rise eventu- 

 ally to the entire nervous system and its 

 associated parts. The retina of the eye forms 

 by an evagination or outpocketing of ecto- 

 derm from the brain and the lens from an 

 invagination of the overlying ectoderm of 

 the head. Evaginations from the wall of the 

 digestive tract give rise to most of the lungs, 

 the liver, and the pancreas, as shown in the 

 human embryo ( Fig. 23-7 ) . 



Referring again to the chick embryo 

 (Fig. 23-8), the orgin of the principal or- 

 gans can be made out after 55 hours of 

 incubation. The three primary brain vesi- 



Fig. 23-6. The nervous system of Amphioxus and of all 

 chordates is formed from a groove along the top 

 side of the embryo, as illustrated by these figures 

 from top to bottom. The mesoderm gives rise to the 

 somites and lining of the coelom. 



