40 



HUMAN EMBEYOLOGY. 



Shortly after it has appeared, each limb bud assumes a semilunar outline ; it 

 projects at right angles from the surface of the body, and it possesses dorsal and 

 ventral surfaces, and cephalic or preaxial, and caudal or postaxial borders. The 



FIG. 51. VIEW OF DORSAL ASPECT OF A 

 HUMAN EMBRYO 1*38 mm. LONG, before 

 the . appearance of the limbs. (From 

 Keibel and Elze, Normaltafeln.) 



FIG. 52. DORSAL LATERAL VIEW OF 

 A HUMAN EMBRYO 2*4 mm. 

 LONG. The Wolffian ridge is seen 

 at the lateral border of the meso- 

 dermal somites. (Keibel and 

 Elze, Normaltafeln.) 



bud is the rudiment of the distal segment of the future limb, the hand in the case 



of the fore-limb, and the foot in the case of the hind-limb. 



As the limb-rudiment increases in length the more proximal segments of the 



limb are differentiated, the forearm and arm in the case of the fore-limb, and the 



leg and the thigh in the case of 

 the hind -limb. At the same 

 time the limbs are folded ven- 

 trally, so that their original 

 ventral surfaces become medial 

 and their original dorsal surfaces 

 lateral, and the convexities of 

 the elbows and knees are directed 

 laterally. At a later period, on 

 account of a rotation which takes 

 place in opposite directions in the 

 fore- as contrasted with the hind- 

 limbs, the convexity of the elbow 

 is turned towards the caudal end 

 of the body and that of the knee 

 towards the cephalic end. It is 

 only at much later periods of de- 

 X velopment, as the erect posture is 



assumed, that the convexity of the 



FIG. 53. LATERAL VIEW OF A HUMAN EMBRYO 2-1 mm. greatest elbow is directed dorsally and the 

 length, showing limb buds projecting from the Wolffian ridge. pnTlv f jV o f the knee ventrallv 

 (Keibel and Elze, Normaltafeln.) Onvexity C illy. 



The terminal or distal seg- 

 ment of each limb is, at first, a flat plate with a rounded margin, but it soon differenti- 

 ates into a proximal or basal part and a more flattened marginal portion. It is 

 along the line where these two parts are continuous that the rudiments of the digits 

 appear. They become evident as small elevations on the dorsal surface of the limb 

 bud about the fifth week ; they extend peripherally, and by the sixth week the 

 fingers project beyond the margins of the hand segment, but the toes do not attain 

 to a corresponding stage of development until the early part of the seventh week. 



