535 



A rabbit embryo of 50 mm in length has arrived at about the 

 same stage of development, so far as the inter-phalangeal sesamoid is 

 concerned, as has a pig embryo of 80 mm. There is about the same 

 amount of initial chondrification of the sesamoid, and the joint-cavity 

 between the second and third phalanges and between the second pha- 

 lanx and the sesamoid, is present. Moreover, ossification has begun 

 in the cartilage of the third phalanx. 



The process of sesamoid development being practically the same 

 in the pig and in the rabbit, a further detailed description of its pro- 

 cesses in the latter animal is superfluous. It is sufficient to say that 

 in the sesamoid of the rabbit at the time of birth a cartilaginous An- 

 lage is present; which, however, is not so typically cartilaginous as 

 is the sesamoid of the pig measuring 150 mm in length. Apparently 

 the main, and practically the only, difference in the embryonic sesa- 

 moid of the pig and the rabbit is its closer connection with the second 

 phalanx in the latter as compared with the former. In this respect 

 the rabbit resembles the sheep. 



The main points revealed by the present investigation are as 

 follows. In all the animals examined the flexor tendon is well ad- 

 vanced in development before there are any indications of the forma- 

 tion of cartilage in the position ultimately occupied by the sesamoid 

 bone. The process of development of the sesamoid is the same, in 

 essentials, in ungulates and in the rabbit. Any differences which exist 

 lie in the closeness of the connection between the rudiment of the 

 sesamoid and the adjacent phalanges. At no period is the sesamoid 

 rudiment intimately connected with the flexor tendon; i. e. the sesa- 

 moid is certainly not intra-tendinous at any time. On the contrary, 

 from the very first, it is very clearly peri-articular in character. 

 That it is so related to the articulation in the adult is fully re- 

 cognised, and it has been so considered from the time of the publi- 

 cation of Gillette's paper at least. 



It seems not unreasonable to conclude that the inter-phalangeal 

 sesamoid of ungulates and the rabbit represents an ossification in a 

 ligament homologous with the glenoid ligament of the distal inter- 

 phalangeal articulation of Man. Its intimate association with the 

 ligament is demonstrated by the accompanying figures. If it were 

 the case that the sesamoid were present in digitigrade animals only, 

 it might be assumed that its presence could be accounted for by the 

 position of the digit in relation to the line along which the weight 

 of the body is transmitted. Seeing, however, that the bone is present 

 in plantigrade animals, it follows that such an explanation is not en- 



