280 - Multicellular Animals, Especially Man 



two distinct layers (Fig. 15-9). The outer 

 layer, or somatic mesoderm, together with 

 the ectoderm around it, becomes the body 

 wall of the adult; and the inner layer, or 

 visceral mesoderm, together with the endo- 

 derm, becomes the gut wall of the adult. 



All triploblastic animals except the flat- 

 worms possess a coelom, although the coelom 

 does not always rise in the same manner. 

 The coelom surrounds the digestive tract 

 and separates it from the body wall. This 

 arrangement permits the digestive tract to 

 move freely — independently of the move- 

 ment of the body as a whole. Generally the 

 archenteron continues to invaginate, until it 

 establishes communication with the exterior 

 at the opposite end of the embryo. In this 

 way the digestive tract becomes a tubular 

 passage, leading from the month to the amis. 

 Accordingly, the body structure of most 

 higher animals can be described as that of 

 a "tube within a tube." The inner tube, 

 which is the enteron, or digestive tract, is 

 separated from the outer tube, or body wall, 

 by the coelom, which extends throughout 

 most of the length of the body. 



In many invertebrates, such as starfish and 

 other Echinodermata (p. 660), the mesoderm 

 and coelom arise from a pair of hollow 

 evaginations, the enterocoelic pouches (Fig. 

 15-2). These outpocketings pinch oft from 

 the archenteron and grow larger until they 

 obliterate the blastocoel. Meanwhile the 

 cavity of each pouch encircles the enteron, 

 forming the definitive coelom. 



A similar method of mesoderm formation 

 is also found in Ampliioxus (p. 665) and 

 some other primitive Chordata (p. 664); but 

 the mesoderm arises in a different way, in 

 embryos of true vertebrates. In the frog, for 

 example, most of the mesoderm is formed by 

 a proliferation of the cells of the rim of the 

 blastopore — that is, at the junction between 

 the ectoderm and endoderm (Fig. 15-8B.C). 

 However, some mesoderm seems to bud off 

 from the sides of the archenteron. 



Regardless of origin, the mesoderm grows, 

 crowding forward in the embryo and insinu- 



ating itself between the ectoderm and endo- 

 derm along both sides of the archenteron. 

 Then the coelomic cavity appears (Fig. 15- 

 8E) and enlarges (Fig. 15-8F), finally en- 

 circling (Fig. 15-8H) the enteron, except in 

 the mid-dorsal region. Thus the coelom 

 comes to separate the visceral and somatic 

 layers of the mesoderm; and the body wall 

 becomes differentiated from the gut wall. In 

 the dorsal region, however, the opposite parts 

 of the developing coelom fail to meet each 

 other above the enteron, although ventrally 

 the coelom completely encircles the gut (Fig. 

 15-8 J). Owing to this failure of the coelom 

 to encircle the gut in the dorsal region of 

 the vertebrate embryo, the gut wall remains 

 in continuity with the body wall — by way of 

 a sheetlike connection of mesoderm tissue 

 (Fig. 1 5-8 J). This connecting sheet of meso- 

 derm becomes the mesentery, which suspends 

 the digestive tract in the coelomic cavity of 

 the adult. Also the mesentery provides a 

 connection through which nerves and blood 

 vessels pass between the both 1 wall and the 

 gut wall, which otherwise are isolated 

 throughout the length of the body. 



FURTHER DEVELOPMENT- 

 VERTEBRATE ANIMALS 



All vertebrates (fish, amphibians, reptiles, 

 birds, and mammals) resemble each other as 

 to many characteristics of development and 

 structure (p. 668); but three vertebrate char- 

 acteristics are of primary importance. All 

 vertebrates possess: (1) a segmented vertebral 

 column (also called the "backbone"); (2) a 

 dorsally placed tubular nervous system (that 

 is, a tubular brain and spinal cord that are 

 enveloped by the vertebral column); and (3) 

 gill clefts (several, usually five, pairs of slit- 

 like channels, leading from the digestive tract 

 to the exterior, in the region of the pharynx). 



Among the fish, the gill arches bear func- 

 tional gills, which serve as respiratory organs. 

 But in higher vertebrates, including man, 

 the gill clefts appear only transiently during 

 embryonic development — although modified 



