534 



embryo of 70 mm in length it appears that chondrification is being 

 initiated in the centre of the condensation. In the same embryo the 

 joint-cavity between the second and third phalanges has been formed, 

 as has also the bursal cleft between the rudiment of the sesamoid 

 and the flexor tendon. In a slightly older embryo (75 mm) cartilage- 

 formation is undoubtedly proceeding. So far as the material at my 

 disposal allows of comparison, it appears that at no time is there so 

 close a connection between the sesamoid rudiment and the third pha- 



^ lanx in the sheep as there 



is, from the earliest period, 

 in the pig. On the other 

 hand, the rudiment is ap- 

 ft- parently much more in- 

 timately connected with the 

 second phalanx in the sheep 

 than it is in the pig. In 

 the horse it seems that the 

 sesamoid rudiment is not 

 very closely joined to either 

 the second or the third 

 phalanx. Suitable material 

 being scanty, however, it is 

 not possible to make any 

 very definite statement on 

 this head. 



That the mode of de- 

 velopment of the inter-pha- 

 langeal sesamoid is the same 

 in the rabbit and in the 

 ungulates became evident 

 early in the comparison. In 

 a rabbit embryo 42 mm in 

 length the condition closely 

 resembles that of the 52 mm pig. There is approximately the same 

 amout of mesodermic condensation between the second phalanx and 

 the flexor tendon ; and, as in the pig, it is closely associated with the 

 third phalanx. In the same specimen some perichondrial ossification 

 is present about the extremity of the third phalanx, and the cartilage 

 cells of the phalanx are enlarged and widely separated. The Anlagen 

 of the metacarpal sesamoids reveal a certain amount of chondri- 

 fication. 



Fig. 5. Pig embryo. 150 mm long (70 days). 

 Longitudinal section through the third digit. Let- 

 tering etc. as in Fig. 4. 



