264 Vertebrate Embryology 



The limbs. During this day the limbs in- 

 crease considerably in size, and become marked 

 off into two parts: a more rounded proximal 

 portion, and a somewhat expanded extremity. 

 In the expanded extremity the digits are be- 

 ginning to be outlined in cartilage, and the 

 rounded proximal portions are slightly bent to 

 form the first indications of the elbow- and 

 knee-joints. 



The angles of the elbow and knee are at 

 first directed almost straight out from the 

 body, but at about the eighth day both limbs 

 rotate, so that the elbow-joint now points 

 backwards, while the knee-joint is directed 

 forwards (Fig. 72, B). As the result of this 

 rotation, the digits of the anterior appendage 

 are pointed directly forwards, while those of 

 the leg are directed backwards. About the 

 tenth day, however, the hand and foot ro- 

 tate on the arm and leg, so that the digits of 

 the former are directed backwards, and the 

 digits of the latter forwards. By the end of 

 the tenth day, the appendages have the out- 

 lines of well-developed wings and legs, except 

 for the absence, as yet, of the feathers and nails. 



The bony skeleton is early outlined in car- 

 tilage, which later becomes ossified. In the 



