384 
On the SesamoifU of the Knee- Joint 
much the same course as that shown in the drawing of Didelphys, but it lacked any 
cyamella where it passed over the head of the fibula ; it arises as in Ornithorhynchxis 
from the fibular crest and not from the external condyle of the femur. The 
expansion of the fibula is not quite so marked as in Ornithorhynchus, but from it 
arises not only poplitens, but the larger or external head of gastrocnemius. The 
internal head is much smaller and arises from the internal condyle*. 
(20) Centre of Ossification of Fibular Crest in ISIonotremes. 
In order to grasp how the fibular crest can be separated from the fibula and 
ultimately be found disintegrated as vestigial sesamoids attached to gastrocnemius, 
2)oj)liteus and plantaris, it is of some importance to determine whether this process 
at the head of the fibula has a separate centre of ossification. 
Parker and Haswellf appear to have no doubt on the matter; writing of the 
Monotremes they say : 
The filiula has at its proximal end a remarkable compresseil process which ossifies from a 
separate centre and greatly resembles the olecranon of the ulna. 
Through the great kindness of Dr Wm. Colin Mackenzie of the Australian 
Institute of Anatomical Research, Melbourne, we were able to obtain sections of 
the fibula both of an embiyonic and a young platypus. He also sent us the section 
of the parafibular sesamoid of a young wombat which had just left the pouch. 
Fig. A, on p. 38.5, indicates the bone of the young platypus set against that of an 
adult platypus to give the relative sizes. The length of the former animal from tip 
of bill to cloacal sphincter was 31 cms. as against the similar length of a fully 
developed adult of 44 cms. The length of the embryonic platypus was 13 cms. 
Fig. B shows the position of the sections Dr Mackenzie made of the embryonal and 
young platypus. 
Taking the section A of the embryo (see our Plate XXXVII, Fig. 96), we have 
periosteum at the proximal end of the fibular process (top of plate), then follows a 
small area staining yellow and suggesting ossification, then an area of cartilage with 
every appearance of ossification starting, and then cartilage again. This certainly 
seems to accord with a centre of ossification in this fibular process or crest. In the 
young platypus in the corresponding section A we found no cartilage at all but bone 
covering the areas where in the embryo we had found cartilage and ossification 
taking place. It vvas therefore probably not young enough to show an intermediate 
partly ossified stage. On the other hand the section towards the top of the crest 
(Plate XXXVII, Fig. 95) shows fairly numei'ous Haversian canals which become 
less frequent or cease altogether as we pass down the section, to become frequent 
* Mivart ("Anatomy of Echidna hystri.r," Tram. Linnean Soc. 1866, p. 394) mentions only what 
appears to be the inner head of gastrocnemius, and probably our outer head is his soh'us, which like his 
popHteus arises from the process at the top of the fibula. 
t A Text-hook of Zoology, Vol. ii. p. 489, Loudon, 1897. The comparison of the olecranon with the 
tibial crest seems more reasonable, the fibular crest should be homologous with a radial rather than an 
ulnar process. The fibular crest of the Mouotremes or that of certain Marsupials has little general 
resemblance to the olecranon, while the tibial crest in birds {Coli/mbus) or the tibia plus patella (as in 
Hesperornis) is very similar to the olecranon in appearance. 
