DEVELOPMENT OF PELVIO GlliDLE IN THE CHICK. 405 



and all other cartilaginous stages. But, since it lies in a 

 somewhat different plane from the rest of the girdle, their 

 junction is only visible in a few sections. In most, the region 

 of junction is not cut through, as appears in fig. 10, which 

 represents a section taken from the series out of which fig. 4 

 was compounded. I think that Bunge must have been misled 

 by the frequent occurrence of such sections, and so have over- 

 looked the few in each series in which the junction is really 

 visible, such as that represented in fig. 11. Fig. 12 again 

 represents a single section, showing the complete continuity of 

 pubis and girdle. It is the ossification alone which gives rise 

 to any want of continuity in any part of the girdle. 



Homologies of the pubis in the different Vertebrate 

 groups. — In tlie pelvic girdle of Ornithorhynchus (see fig. 17) 

 a large process, whose length is about three quarters of that of 

 the pubis, projects forwards from the region in front of the ace- 

 tabulum, in bony continuity with the pubis. The same process 

 is found in a somewhat reduced form in Echidna, and is still 

 more reduced in many Marsupials and higher Mammals. 

 Sometimes it is entirely absent. In embryo birds (see fig. 15), 

 the process is found in about the same proportionate condition 

 of development as in Ornithorhynchus. In the adult, it 

 becomes much reduced, or is absent. Sometimes, as in the 

 Ostrich, the ilium takes a small share in its formation, but 

 this appears to be a secondary condition. It is the pectineal 

 process of the pubis. 



In the Dinosaurs, as described first by Marsh, ^ the embryonic 

 condition of birds and the adult form of Ornithorhynchus is 

 preserved in the almost equal development of the two branches 

 of the pubis, the anterior being shorter and more massive, and 

 the posterior longer and more slender (fig. 16). 



The homologies in these cases seem clear, and have been 

 generally recognised. 



Turning to the reptiles, it is easy to compare the pubis of 



' O. C. Marsh, " Principal characters of American Jurassic Dinosaurs," 

 ' American Journal of Science and Arts' (Sillimaii), vols, xvi and xvii, 187S 

 and 1870. 



