IV. YEHTEBRATA. 



529 



is usually believed that all tiiese funiis are to be traced back to 

 an ancestral type, the archipterygiuin. In this (fig. 5<;3) are 

 numerous skeletal parts which ^'ary little in size and form and are 

 arranged in many closely appressed rows. One of the rows lias 

 acquired prominence and is called the principal row; it begins 

 with a larger piece, the metapteryginm, which articulates with the 

 girdle aiid bears either on both sides {archip)terygium biseriale) or 

 only on one (archipterygium uuiseriale) the lateral rows of skeletal 

 elements. Usually most of the lateral rows are not attached to 

 the pjrincipal row, but arise independently from the girdle, and 

 may begin with larger parts, the propterygium and mesopterygium. 



From this archipterygium can be derived a jjrimary form which 

 serves for all terrestrial vertebrates from the 

 Amphibia onwards; it is the pentadactyle ap- 

 pendage (fig. 565). In tracing this from the 

 archipterygium (of either uniserial or biserial 

 type) the following modifications must be sup- 

 posed. First a reduction in the number of 

 rows to five, a principal row and four acces- 

 sory rows. The terminal portions of the prin- 

 cipal row produce the bones of the fifth, the 

 accessory rows of the other iingers. Tlien 

 there is an unequal growth of parts; the meta- 

 pteryginm, already in Elasmobranchs a con- 

 siderable element, increases in size and forms 

 in the fore limb the humerus, in the hind 

 lunb the femur. In like manner the second 

 element of the principal row and the first of 

 the first accessory row increase and form re- 

 spectively ulna and radius in front, fibula and 

 tibia behind. Then follow parts which remain 

 small and somewhat cubical, carpal bones in 

 the fore limb, tarsals in the hinder extremity; 

 they bear in turn slender bones, the meta- 

 carpals or metatarsals, and these at last the 

 phalanges. (For the nomenclature of carpals 

 and tarsals see the explanation of fig. 505.) 



The third and most important modification is brought about 

 by the development of joints. So long as the appendage served 

 as an oar it must act as a single plate with its parts firmly held. 

 On the other hand, when it must act as a system of levers to sup- 

 port and move the body, as is necessary in a terrestrial animal, it 



Fig. .565.— Schema of a 

 pentadactyle appen- 

 dage. (After Gegen- 

 baur ) The dotted 

 lines indicate the lat- 

 eral rays ; the namea 

 for the hinder ex- 

 tremities in parenthe- 

 ses. H, humerus (fe- 

 mur); U, ulna (fibula): 

 H, radius (tibia). Car- 

 pus (tarsus) consist- 

 ing of two rows and 

 two central portions: 

 Row I: r, radiale (tibi- 

 ale): /.intermedium; 

 %i, uinare (fibularei: c, 

 centralia. Row II: 

 i-5,carpalia (tarsalia); 

 the metacarpals (met- 

 atarsals) and pha- 

 langes not lettered. 



