533 



thus the approach of cartilage formation is indicated. The cells around 

 the area are concentrically arranged, and the area itself is sharply 

 defined from the perichondrium of the third phalanx. 



An embryo 93 mm in length shows only a slight advance upon 

 the condition of the 88 mm embryo. The cartilage formation in the 

 Anlage of the sesamoid is slightly more evident (Fig. 4, s), and there 

 is a small amount of bony deposit under the perichondrium about the 

 middle of the second phalanx. 



b.- 



f.t.- 





■mimm 



Fig. 4. Pig embryo. 93 ram long (53 days). Longitudinal section tlirough the 

 distal inter-phalangeal articulation of the third digit. 2, 3, and f.t. as in Fig. 2. 

 s. Anlage of sesamoid, b. bursa between the flexor tendon and the sesamoid. 



Embryos measuring 100 mm and 115 mm reveal the appearance 

 of a strip of loose tissue between the sesamoid and the third phalanx, 

 in which there is a faint indication of the production of a fissure which 

 finally extends the joint-cavity between the two bones. The bursa 

 associated with the flexor tendon is more extensive. 



The only other embryo examined is one (150 mm long) in which 

 the adult condition is approximated so far as form is concerned. The 

 sesamoid is fully formed in cartilage, and the joint-cavity between the 

 second and third phalanges is continued between the sesamoid and 

 the third phalanx (Fig. 5). 



Those embryos of the sheep and the horse which have been 

 available for examination reveal, in general, the same steps in the 

 development of the interphalangeal sesamoid as have been related 

 above as occurring in the pig. 



In a sheep embryo 55 mm long the mesodermic condensation in 

 the position of the future sesamoid is very conspicuous; and in an 



