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

 By K. Broom, M.D., B.Sc. Communicated by Professor Sir Wm. Turner. 

 (With Two Plates.) 



(Read January 9, 1899.) 



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. 



In studying the conditions in marsupial embryos much earlier than any that had 

 hitherto been examined, I have been fortunate in discovering one or two facts which 

 throw considerable light on the changes which have taken place in the course of the 

 modification of the highly specialised shoulder girdle of the Anomodonts into the simple 

 and more rudimentary condition met with in the higher mammals. 



Some time ago I published a short paper (1) recording the occurrence in the newly- 

 born Trichosurus of a well-developed coracoid which articulates with the sternum. 

 Since then I have traced the course of development from the very early and rudimentary 

 condition met with in an intra-uterine embryo of Trichosurus of 8 "5 mm., onward till the 

 adult condition is reached. 



Before entering on a discussion of the various views that have been advanced 

 regarding the morphology of the various elements, it will be well to describe the course 

 of development of the structures in the marsupial. In the following descriptions I 

 nave made use of the terminology which seems to me the most satisfactory, reserving 

 for the latter part of the paper the reasons for the various opinions. 



VOL. XXXIX. PART III. (NO. 29). 6 A 



