16 



MORPHOGENESIS 



each segment, or Jiephrotome, gives rise to a portion of the mesonephros, the provisional kidney. 

 Other cells of the mass become inesendujme, which is converted into blood-vessels, connective 

 tissue, etc. 



As development proceeds, the metamerism of the muscles and arteries becomes more or less 

 obscured, but that of the vertebra? and nerves is fully retained even in the adult. In the case of 

 the muscle plates, from which all the voluntary musculature of the trunk is derived, great modi- 

 fications occur. Extensive fusion of successive plates occurs, the intervening connective tissue 

 disappearing more or less completely; associated with this fusion there is longitudinal and tan- 

 gential splitting of the somites to form individual muscles; and portions of some of the plates may 

 wander far from their original position. But notwithstanding these comphcated changes, in- 

 dications of the primary metameric arrangement of the muscle plates are abundant, and even in 

 the most extreme cases of modification the developmental history of a muscle can be determined 

 by means of its nerve supply. For the fibres derived from each plate will usually retain, no 

 matter what changes of independence or position they may undergo, the innervation by their 

 originally corresponding segmental nerve; so that the occurence in the lumbar region of the body 

 of muscle-fibres (the diaphragm) supplied by nerve-fibres from a cervical nerve is evidence that 

 the muscle-fibres have been derived from a cervical mesodermic somite and have subsequently 

 migrated to the position they finally occupy. 



As regards the arteries, they arise primarily from a longitudinal stem, the aorta, in a strictly 

 segmental manner, each metamere having distributed to it two pairs of arteries and a single 

 median one (fig. 20) . One pair of arteries supphes the body wall, and these retain very distinctly 

 their original metameric arrangement; the other pair passes to the paired viscera, such as the 

 lungs, kidneys, ovaries (or testes), so many of the pairs disappearing, however, that their meta- 

 meric arrangement is not very evident in the adult. The unpaired vessels supply the digestive 

 tract and its unpaired appendages, such as the liver and pancreas, and undergo great modifica- 

 tions, those of the lower thoracic and lumbar regions becoming reduced by fusion and degenera- 

 tion to three main trunks. 



Fig. 20. — Diagram of a Transverse Section through the Abdominal Region. 



DORSAL MUSCLES fjm-. DORSAL MUSCLES 



K I DNEY 



^ 



PERITONEUM 



V ^ \ PARIETAL LAYER 



\ " -^ 



\ v\ 



\, 



Branchiomerism. — Throughout the trunk and neck regions, then, a funda- 

 mental metameric plan underlies and determines the arrangement of many parts. 

 In the head there is also evident a primary arrangement of the parts in succession; 

 but this arrangement appears to be somewhat different from that of the trunk in 

 that it involves the v(;ntral instead of the dorsal mesoderm and is associated with 

 the occurrence of branchial arches rather than with true mesodermic somites. 

 It is consequently termed hranchiornerUm. 



Not but that there are also indications of metamerism in the head, the muscles of the orbit, 

 and the majority of the oxtrinsic muscles of the tongue, together with the nerves supplying 

 these muscles, l)eing apparent ly metameric structures, but the metamerism of this region of the 

 body is largely overshadowed by the branchiomerism. 



If an embryo of about the fifth week of development (fig. 22) be examined, there will be 

 observed on the surface of the body in^the pharyngeal region three or four linear depressions, 



