IX.] OSSIFICATION. 283 



toes. These on the sixteenth day become harder and 

 more horny, as does also the beak. 



Nails are developed on special regions of the epidermis, 

 known as the primitive nail beds. They are formed by the 

 cornification of a layer of cells which makes its appearance 

 between the horny and mucous layers of the epidermis. The 

 distal border of the nail soon becomes free, and the further 

 growth is effected by additions to the under side and attached 

 extremity of the nail. 



By the thirteenth day the cartilaginous skeleton is 

 completed and the various muscles of the body can be 

 made out with tolerable clearness. 



Ossification begins according to Von Baer on the 

 eighth or ninth day by small deposits in the tibia, in 

 the metacarpal bones of the hind-limb, and in the sca- 

 pula. On the eleventh or twelfth day a multitude of 

 points of ossification make their appearance in the 

 limbs, in the scapular and pelvic arches, in the ribs, in 

 the bodies of the cervical and dorsal vertebrae and in 

 the bones of the head, the centres of ossification of the 

 vertebral arches not being found till the thirteenth day. 



The events which we have thus briefly narrated are 

 accompanied by important changes in the arterial 

 and venous systems. 



The condition of the venous system at about the 

 end of the third day was fully described in Chap. vi. 

 p. 170, and the changes which have taken place between 

 that date and the latter days of incubation may be seen 

 by comparing the diagram Fig. 58 with the diagrams 

 Figs. 89 and 90. 



On the third day, nearly the whole of the venous 

 blood from the body of the embryo was carried back to 



