PBVELOPMENT OF PELVIC GIRDLE IN THE CHICK. 15 



outgrowths of the proximal part of the femur, one being directed 

 upwards, the other inwards, each, however, hardly extending 

 beyond the limb itself. The future cartilage is only just dis- 

 tinguishable from its surroundings of indifferent mesoblastic 

 cells, since the two tissues pass quite gradually into one 

 another. 



The next stage (length of hind limb 0"13 in.) is seen in 

 longitudinal section in fig. 2. The series of sections shows the 

 same perfect continuity of the girdle and femur that existed at 

 first. We can distinguish in the girdle a blunt dorsal prolonga- 

 tion — the beginning of the ilium — an acetabular region behind 

 the obturator nerve and a downward process in front of it, 

 which is obviously to become the pubis. As we go inwards in 

 the series of sections these two outgrowths, the ilium and the 

 pubis, disappear, and the central or acetabular region is pro- 

 longed a little way inwards, being bounded in front by the ob- 

 turator nerve. The nerve does not appear in the same sections 

 with the pubis and ilium, but in the figure it is represented as 

 viewed from the outside, the girdle being supposed to be trans- 

 parent. At this stage the nerves are remarkable for their large 

 size in proportion to the skeletal parts. The obturator nerve 

 coming off from the crural plexus is at this time by far the most 

 important of its distal branches. 



In the next stage (length of hind limb 0*15 in. — see fig. 3) 

 we can clearly distinguish three elements in the girdle, meeting 

 in the broad acetabular region, which passes on Avithout a break 

 into the femur. The region of its junction with the latter is 

 shown diagrammatically in the figure, but the cartilage of the 

 femur is continuous with that of the girdle, as are the three 

 elements of the girdle with one another. The ilium has grown 

 forwards, arching over the crural nerve, and has given off a 

 slenderer pointed process backwards. The ischium is directed 

 almost vertically downwards, but also slightly inwards, being, 

 as a whole, situated nearer to the middle line of the body than 

 are the other elements. The main point of interest is the double 

 nature of the pubis, the anterior branch of which points directly 

 forwards and slightly outwards, while the posterior is directed 



