530 



stitutes a not inconsiderable component of the digit; and, further, a 

 complete series of embryos of one ungulate — to wit, the pig — was 

 obtainable without great difficulty. In order that the investigation 

 might include a member of some order other than that of the Ungu- 

 lata, embryos of the rabbit have also been microscopically examined. 

 Sheep and horse material has likewise been used, but a complete 

 and suitable series of embryos of these animals was not procurable. 

 From the specimens which were available, however, it was clear that 

 the development of the sesamoid follows, in the main, the same lines 

 in the pig, the sheep and the horse. 



In all, ten developmental stages have been investigated in pig 

 embryos ranging from 32 mm to 150 mm in length. Of all these 

 longitudinal sections of the manus have been made ; and of five of them 

 transverse sections were also examined. 



In a 32 mm pig embryo the first and second phalanges are re- 

 presented by nodules of cartilage surrounded by thick capsules of 

 condensed mesoderm; but the third phalanx is not as yet truly carti- 

 laginous in any part (Fig. 1). Its position, however, is clearly indicated 

 by a condensation of the mesoderm, the cells of which are showing 

 some alteration of character — foreshadowing future cartilage formation 

 — in the more central and proximal part of the mass. The Anlage 

 of the flexor tendon is very clearly and sharply defined; and consists 

 of cells of oval outline closely packed together with their long axes 

 conformable to the direction of the future tendon. 



Between the second and third phalanges there is a fairly wide 

 gap filled by mesoderm of a loose character. In an area lying between 

 the two phalanges and the flexor tendon, the mesodermic cells are 

 rather more numerous. This is the position in which the sesamoid is 

 subsequently formed (Fig. 1). 



The third phalanx of a 37 mm embryo is not yet definitely carti- 

 laginous, but the more central cells of the tissue composing its rudiment 

 are larger than the rest and separated from each other by appreciable 

 intervals. In the same manus the mesodermic condensation between 

 the flexor tendon and the second and third phalanges is more pro- 

 nounced than it was in the smaller embryo. 



In a 43 mm embryo the third phalanx is very definitely carti- 

 laginous, and more especially so at its most distal part; thus forecasting 

 the point at which ossification in the cartilage will first occur, and the 

 direction in which it will spread. It is noteworthy that, in the same 

 position, there is a trace of osseous deposit formed by the perichond- 

 rium. The mesodermic condensation, previously found between the 



