284 - Multicellular Animals, Especially Man 



the endoderm provides only the internal 

 epithelium of the digestive tract and its 

 branches. This epithelium is particularly 

 important in the case of the liver, pancreas, 

 and other digestive glands, because only 

 these endoderm cells are destined to give rise 

 to glandular tissues. 



Origin of Some Other Structures. The 

 blood and lymph vessels originate as fissures, 

 called sinuses, in the mesoderm (Fig. 15-8, J), 

 and these sinuses eventually become inter- 

 connected, forming a continuous system of 

 tubes. At first, the heart appears as a simple 

 tube formed by the coalescence of two of the 

 larger vessels in the anterior ventral region 

 of the embryo (Fig. 17-8). This primitive 

 heart soon becomes muscular and begins to 

 beat regularly, and thereafter the heart con- 

 tinues to perform its ceaseless work while 

 further development goes on. The initially 

 straight tube becomes twisted rather com- 

 plexly (Fig. 17-8), and meanwhile partitions 

 are formed, first between the auricles, and 

 then between the ventricles. In the embryonic 

 mammalian heart, therefore, it is possible to 

 recogni/e stages that are equivalent to the 

 two-chambered heart of the fishes and the 

 three-chambered heart of the amphibians, 

 before finally the four-chambered heart of 

 the adult mammal comes into being. 



In the early embryo the blood plasma ap- 

 pears to be secreted into the sinuses by the 

 surrounding epithelial cells, and both red 

 and white corpuscles arise from cells that 

 detach themselves from the sinus epithelium. 

 Later, however, the formation of new cor- 

 puscles becomes delegated to other tissues, 

 particularly the bone marrow (p. 322). 



The kidneys and the gonads (ovaries or 

 testes) originate from mesodermal foldings 

 along the dorsal wall of the coelom, on either 

 side of the mesentery (Fig. 15-1 IE). In the 

 embryo of reptiles, birds and mammals, 

 moreover, three stages may be recognized 

 during the development of the kidneys. The 

 earliest embryonic kidney, called the pro- 

 nephros, occupies a relatively anterior posi- 

 tion in the embryo. In structure this primi- 



tive kidney resembles the adtdt kidneys ol 

 present-day fishes. In higher vertebrates the 

 pronephros never becomes functional and is 

 soon replaced by a functional mesonephros, 

 which is situated more posteriorly and plainly 

 displays a structural resemblance to the adtdt 

 amphibian kidney. Finally, the mesonephros 

 is likewise replaced by the adtdt type of 

 kidney, the metanephros, which develops 

 near the posterior extremity of the coelomic 

 cavity. 



Contributions of the Primary Germ Layers. 

 In summary, the eventual contribution of 

 each germ layer, especially in vertebrate 

 animals, can be given as follows: 



The ectoderm gives rise to: (1) the entire 

 nervous system — including the brain, spinal 

 cord, ganglia, nerves, and the receptor cells 

 of the sense organs: (2) the outer, or epider- 

 mal layers of the skin, and skin structures — 

 including hair, trails, the superficial parts of 

 scales and feathers, and the secretory cells of 

 skin glands (for example, sweat glands); (3) 

 the lens of the eye; and (4) part of the lining 

 of the mouth, nostrils, and anus, including 

 the enamel layer of the teeth. 



The endoderm provides the lining of the 

 enteron (excluding part of the mouth and 

 anus) and all its offshoots. These offshoots 

 include not only the digestive glands such as 

 the liver and pancreas, but also the lungs 

 and other parts of the respiratory tract, the 

 thyroid and thymus glands, and most of the 

 urinary bladder and the urethra. 



Finally, the mesoderm gives rise to all 

 other body structures. These include (1) the 

 deeper (dermal) layers of the skin and asso- 

 ciated structures: (2) the gut wall, except for 

 its enclodermal lining; (3) the mesenteries 

 and other parts of the epithelial lining of the 

 coelom and its derivatives; (4) the kidnevs 

 and most of the reproductive system; (5) the 

 vascular system, including the heart, blood 

 vessels, and lymphatics; (6) the muscles; 

 and (7) the various connective and sup- 

 porting tissues, including almost all the 

 bones, and the deeper (dentine) layers of the 

 teeth. 



