ORGAN DEVELOPMENT IN VERTEBRATES 263 



skill. Somewhat unexpectedly at first sight, the boundaries between the 

 vertebrae do not correspond to the grooves between the somites ; in each 

 vertebra of the adult the posterior half has been produced by the anterior 

 part of one somite while the anterior half is derived from the posterior 

 part of the somite next forwards in the series. This arrangement ensures 

 that the muscles arising within each somite are from the beginning joined 

 to two contiguous vertebrae. 



In Amphioxus, and in the primitive vertebrates such as cartilaginous 

 fishes, the somites are hollow (Fig. 12.7). The cavity within them (the 

 myocoel) is part of the general body-cavity, and is continuous with the 

 larger and better-developed space (the splanchnocoel) which forms within 

 the lateral mesoderm. Both cavities together are known as the coelome. 

 In higher vertebrates the myocoels are small and not always easy to 

 detect. It is within the lateral parts of the mesoderm that the main body- 

 cavity of the adult develops; the mesoderm lying above the space be- 

 comes closely applied to the ectoderm, and forms the dermal layer of the 

 skin, while that below lies against the endoderm and produces the 

 muscular layer of the gut. The connection between the upper and lower 

 layers of mesoderm persists in certain places, and provides the mesenteries 

 by which the gut and its derivatives are attached to the main part of the 

 body. 



7. The tail and hind part of the body 



At the stage when, in the amphibian, the neural plate becomes clearly 

 dehmited and the neural folds appear, the greater part of the plate is 

 destined to form the brain and the nervous system of the anterior region 

 of the body. The material for the whole posterior part of the trunk, and 

 for the tail, is concentrated in a small region near the remains of the blasto- 

 pore. In the chick, the material for the brain and anterior end arrives in 

 place, and begins to differentiate, still more in advance of that destined to 

 build the posterior end; and by the time only five or six pairs of somites 

 have appeared, the primitive streak has already become quite short, 

 although the greater part of the trunk is still to be produced. These facts 

 have suggested to some authors, of whom the most authoritative in 

 recent years was Holmdahl (1939), that the gastrulation process as we 

 normally conceive it is responsible only for the formation of the anterior 

 part of the animal and that the posterior part is produced by some radi- 

 cally different process which goes on within the small remnant of blasto- 

 pore or primitive streak. Tliis region, from which the posterior part forms, 

 is knov^Ti as the tail-bud. The authors who argue that the processes going 

 on within it are quite different from those of gastrulation nevertheless 



