16 ALICE JOHNSON. 



downwards and slightly forwards. The obturator nerve passes 

 between the posterior branch of the pubis and the ischium. A 

 series of twelve longitudinal sections has been combined to pro- 

 duce the figure, which is therefore diagrammatic only in so 

 far as it represents as existing in two dimensions what really 

 exists in three. The other figures, in which the whole girdle 

 is represented, were drawn in the same way. 



The study of the stages described above shows that the early 

 development of the Pelvic Girdle of the Chick is similar to 

 that of the limb-girdles of Elasmobranchs ^ in two points : (1) 

 the skeleton of the limb is developed continuously with the 

 girdle ; (2) the parts of the girdle which are in the immediate 

 neighbourhood of the skeleton of the limb are first developed, 

 and the dorsal and ventral outgrowths appear later. 



In the next stage (length of hind limb 0"17 in., see fig. 4) 

 the posterior branch of the pubis has grown more than the 

 anterior, and is curved backwards. Its proximal half, how- 

 ever, retains the direction which the whole posterior branch 

 had in the earlier stage, and from this we may conclude that 

 the change of form results from a growth, rather than from a 

 rotation backwards of the whole cartilage. 



A transverse section (see fig. 5) of about the same stage 

 shows that the girdle is still continuous with the femur. In 

 the latter, the cartilaginous matrix has begun to be formed 

 internally, while the peripheral parts (a region of which is cut 

 through in the middle of the limb) and the girdle still consist 

 of the condensed tissue described above. 



A later stage is shown in fig. 6 (length of hind limb 0'2 in.). 

 The most striking feature here compared with the preceding 

 stage is the large development of the posterior part of the 

 ilium. The ischium has become distally expanded, and the 

 posterior branch of the pubis is larger still in proportion to the 

 anterior branch. 



About this time the femur begins to be separated from the 



girdle by an intervening tract of tissue which has not gone so 



' F. M. Balfour, " Oa the development of the skeleton of the paired fins of 

 Blasmobraachii," 'Proc. of Zoological Society,' 1881. 



