531 



flexor tendon and the phalanges, is now well marked, and particularly 

 so where it is continuous with the perichondrium of the third phalanx. 

 Proximally it is less in amount and, in the neighbourhood of the middle 

 line of the digit, it terminates on a level with the middle of the second 

 phalanx. In this embryo the first trace of cartilaginous rudiments of 

 metacarpo-phalangeal sesamoids is present. 



The third phalanx of an embryo measuring 52 mm in length is 

 completely modelled in cartilage; with the consequence that its peri- 

 chondrium is relatively thin. Although perichondrial osseous deposit 

 has increased somewhat over the distal part of the third phalanx, 

 there is no trace of ossification in any other part of the digit. The 



fM-^:-M...{. 



■- S' 



Fig. 1. 



Fig. 2. 



Fig. 1. Pig embryo. 32 mm long (35 days). Longitudinal section through the 

 third digit. 1, 2, 3 first, second and third phalanges, f.t. flexor tendon. 



Fig. 2. Pig embryo. 52 mm long (40 days). Longitudinal section through the 

 third digit. 2, 3 second and third phalanges, f.t. flexor tendon, s' condensation in 

 the position of the future sesamoid. 



condensed tissue between the flexor tendon and the second and third 

 phalanges has not undergone any appreciable change beyond that of 

 increase in size (Fig. 2, s'). 



An embryo measuring 80 mm in length shows, by the enlargement 

 and separation of the cells, that ossification has begun at the most 

 distal part of the cartilage of the third phalanx, but not in any other 



34* 



