356 EMBKYOLOGY OF THE LOWER VERTEBRATES CH. 



9 



In Birds the pelvic girdle during the prochondral stage passes 

 through the A-shape, the right and left halves distinct from one 

 another and each at first continuous with the skeleton of the limb. 

 Pubis, ilium and ischiuin usually chondrify separately but in many 

 cases (e.g. in the Common Fowl usually) ilium and ischiuin may 

 Income chondrified in continuity, and less frequently all three 

 elements chondrify in continuity. A highly characteristic feature 

 of the avian pelvis is that the pubis swings in a tailward direction 

 about its attached dorsal end until it assumes a position parallel 

 with that of the pubis. In the pelvis shown in Fig. 170, B, this 

 rotation is just commencing. 



The Cheiropterygium (Huxley), or skeleton of the limb in 

 Amphibia and Amniota, consists of three distinct portions corre- 

 sponding respectively to the Upper 



l r il. Arm or Thigh, the Forearm or Leg, 



v ^-^ and the Hand or Foot. As these por- 



tions are looked upon as homologous 

 in the fore and hind limbs it is con- 

 venient to have a morphological name 

 x x;^ for the corresponding parts of the two 

 P B is. se ^ s f limbs, and such names have 

 been proposed by Emery and Haeckel 

 FIG. 170. -Side view of pelvis of Bird _ Stylopodium, Zeugopodium or 



Zygopodium, and AutSpodium. In 



V ^i^^X CMglen; the autopodium there may further 



be recognized Basipodium (carpus or 



tarsus), Metapodium (metacarpus or metatarsus) and Acropodium 

 (phalanges). 



The limb skeleton is typically at first quite continuous. A rod- 

 shaped condensation of mesenchyme appears first in the limb stump 

 the rudiment of the stylopodium (femur or humerus) and as the 

 limb grows this spreads outwards, bifurcating as it does so to form 

 the rudiments of the zygopodial skeleton : with further growth the 

 two limbs of this unite distally to form the rudiment of the auto- 

 podial skeleton. Chondrification takes place from the base of the 

 limb outwards, each separate element of the adult making its appear- 

 ance as a separate chondrification centre. 



The skeleton of the autopodium originates in a flattened plate- 

 like extension of the prochondral zygopodial skeleton. In tin's 

 the various carpal or tarsal elements make their appearance as 

 s" pa rate centres of chondrification. It seems unnecessary in a 

 general text-book like tbe present to go into the great variations in 

 detail which are found amongst the various tetrapods in regard to the 

 skeleton of carpus and tarsus. It, need only be said that the striking 



ions found in different groups from the schematic arrangement, 



such as is illustrated by r-'ig. 171, seem lo have been brought about 

 bv eidai'j'Miienl 01- reduction of individual elements, or ihe fusion 

 of originally separate elements. 



