XXIX.—On the Development and Morphology of the Marsupial Shoulder Girdle

The various structures in the marsupial shoulder girdle are so unquestionably homologous with similar structures in the girdle of the human subject and of the Eutheria generally, that there has never been any difficulty in interpreting the parts in terms of human anatomy; but when the elements are compared with those in what we may consider to have been the ancestral condition, such as is exemplified in the shoulder girdle of the Monotremes, or of the Anomodonts, or in the more distantly related conditions met with in modern Reptiles and Amphibians, we are confronted with quite a number of difficulties, to which the most varied solutions have been applied by different morphologists. The chief difficulty in the way of comparing the typical mammalian girdle with that of the lower forms, is that in the girdle of the higher mammals certain structures are rudimentary, and, further, that the forms that have hitherto been examined have been so far removed from the ancestral stock that even a study of the early developmental conditions fails to give any clues that are more satisfactory than those afforded by comparative anatomy. Though it has been suspected that further results would be obtained from the study of the early condition in Marsupials and Monotremes, the embryos of these groups that have hitherto been studied have been too old to yield much further evidence than that obtained by a study of the adult forms.