DEVELOPMENT OF THE LIMBS. 



743 



The whole thickness of the somatopleure division of the mesoblast is 

 involved in this primary limb-bud, and it is of course also covered 

 with the epiblast or cuticular layer, in the substance of which there 

 is considerable increase of thickness at the most prominent part of the 

 margin. 



As the limb-buds increase in size, the lateral limb-plate, or Wolffian 

 ridge, which is at first very prominent in its whole length, becomes 

 less, and gradually flattens down into the more even surface of the wall 

 of the trunk. 



The part of the limb which appears first, corresponds more imme- 

 diately with the hand or foot than with the other divisions of the limb. 

 Along with this, however, at a very early period there is an indication 

 of the formation of the limb-girdles as folds passing off from the side 



Fig. 542. — Diagrammatic 

 Outline of the Profile 

 VIEW OF the Human Em- 

 bryo OF ABOUT seven 

 weeks, TO SHOW THE PRIMI- 

 tive relations of the 

 Limbs to the Trunk. 

 (Allen Thomson.) f 



r, the radial (preaxial), 

 and u, the iilnar (postaxial) 

 border of the hand and fore- 

 arm ; t, the tiljial (preaxial), 

 and /, the fibular (postaxial) 

 border of the foot and lower 

 leg. (The foot is represented 

 at a somewhat more advanced stage than the rest of the embryo). 



of the trunk. As the projection of the limb increases from the side of 

 the body, the distal or terminal segment becomes slightly notched off 

 from the part next the trunk. This terminal part, forming nearly 

 three-fourths of a somewhat circular flattened plate, contains the rudi- 

 ments of the hand or foot. The next change which takes place is 

 in the division of the proximal part, or rather the preaxial border 

 and ventral surface, by a notch which separates the fore-arm and 

 lower leg from the upper arm and thigh at the elbow and knee joint 

 respectively. In the third stage the notched division of the free lateral 

 curved margin, with intermediate slightly tubercular projections of the 

 substance, shows the commencement of digital development, in which it 

 soon becomes apparent that the pollex and hallux occupy the preaxial 

 position in their respective limbs, and are followed by the series of 

 other fiugers to the fifth, which is placed on the postaxial border. 

 From these it is easy to trace, by reference to the simple original posi- 

 tion of the limbs, the preaxial position afterwards held by the radius in 

 the one and the tibia in the other, and the postaxial position of the ulna 

 or fibula. In the meantime the internal differentiation of texture takes 

 place, by which is brought out the more complete distinction of the 

 segments of the limbs, and the various component parts of each, which 

 gradually appear in the cartilages for the bones, muscular plates 

 extended from the general muscular sheath of the trunk, prolongation 

 of the cutaneous layer of the integument, the formation of nerves, 

 blood-vessels, &c., the consideration of all which belongs to the history 

 of more advanced development. 



