DEVELOPMENT OF PELVIC GIEDLE IN THE CHICK. 399 



On the Development of the Pelvic Girdle and 

 Skeleton of the Hind Limb in the Chick. 



By 



Alice JolBnson, 



Newnham College, Cambridge. 



With Plates XXVI & XXVII. 



The investigations described below were undertaken at the 

 suggestion of the late Professor Balfour, with a view to finding 

 out, through a study of the development of the pelvic girdle in 

 the chick, what are the homologies of the pubis in birds with 

 that of other Vertebrata. 



In connection with this question one or two other points, 

 which appear to me of some importance, have presented 

 themselves. 



The histological development may be briefly considered at 

 the outset. On the fourth day of incubation the limb is merely 

 a local exaggeration of the Wolffian ridge, consisting, like it, of 

 a mass of rounded mesoblastic cells, very closely aggregated to- 

 gether. The epiblast forms a thickened cap round the free end 

 of the limb. No differentiation into cartilage or muscle is yet 

 visible. 



The first trace of the skeletal parts appears on the fifth day. 

 The mesoblastic tissue of the limb is now differentiated into an 

 axial, or more condensed, and surrounding, or less condensed, 

 region. Both parts consist of the same rounded cells as before. 

 They only differ in the degree of concentration of the cells. 

 These features are shown diagrammatically in fig. 1. 



The differentiation of tissue goes on more rapidly in the 



