532 



f.t 



segment of the digit ^). This demonstrates that intracartilaginous os 

 sification begins simultaneously, or almost simultaneously, in the third 

 phalanx and in the shaft of the metacarpal bone; whereas in the 

 other parts of the manus it is slightly later in its onset. The amount 

 of perichondral bony deposit about the tip of the third phalanx is 

 now considerable. 



The condensation in which the inter-phalangeal sesamoid is about 

 to develop is more precisely defined and its cells are beginning to 

 arrange themselves in a concentric manner (Fig. 3, s'). It is, however, 



not yet possible to say 

 that cartilage formation has 

 begun. The metacarpal se- 

 samoids, on the other hand, 

 are well formed in cartilage. 

 It will be noticed from fig. 3 

 that a joint- cavity has ap- 

 peared between the second 

 and third phalanges. 



In an embryo 88 mm 

 long there is a calcareous 

 deposit between the enlarged 

 and separated cells of the 

 cartilage at the distal end 

 of the third phalanx; and 

 in the centre of the first and 

 second phalanges the car- 

 tilage cells are enlarged and 

 separated by considerable 

 intervals. The joint -cavity 

 between the second and third 

 Fig. 3. Pig embryo. 80 mm long (49 days), phalanges is more exteusive 



Longitudinal section through the fourth digit. Letters , . , Tjvpvioim pmbrvo • 

 and figures as in Fig. 2. ''"^" 1" ^^^ preVlOUS emUlJO, 



and a small synovial bursa 

 has appeared between the flexor tendon and the sesamoid condensation. 

 In the condensed tissue itself there is an area, close to the third 

 phalanx, in which the cells are becoming less closely packed together: 



1) The delay in endochondral ossification until after the formation 

 of the perichondral cap of bone agrees with what is known to occur 

 in mammals in general (cf. H. Leboucq, Recherches sur le developpe- 

 ment des phalanges terminales des doigts. Ann. de la soc. de med. de 

 Gand, T. 84, 1904). 



