IV. VERTEBRATA. 



529 



is usually believed that all these forms are to be traced back to 

 an ancestral type, the archipterygium. In this (fig. 563) are 

 numerous skeletal parts which vary little in size and form and are 

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

 acquired prominence and is called the principal row; it begins 

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

 girdle and bears either on both sides (archipterygium biseriale) or 

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

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

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

 may begin with larger parts, the propterygium and mesopterygium. 



From this archipterygium can be derived a primary 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 fingers. Then 

 there is an unequal growth of parts; the meta- 

 pterygium, already in Elasmobranchs a con- 

 siderable element, increases in size and forms 



the fore limb the humerus, in the hind 



in 



limb 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. 565.) 



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 names 

 for the hinder ex- 

 tremities in parenthe- 

 ses. H, humerus (fe- 

 mur); U, ulna (fibula); 

 #, radius (tibia). Car- 



Eus (tarsus) consist- 

 ig of two rows and 

 two central portions: 

 Row I: ?% radiale (tibi- 

 ale); i, intermedium; 

 u, ulnare (fibulare); c, 

 centralia. Row II : 

 l-5,carpalia (tarsalia); 

 the metacarpals (met- 

 atarsals) and pha- 

 langes not lettered. 



