RUDIMENTS OF THE LIMBS. 37 1 



Fio. 124. — Human embryo of five weeks old, opened on the ventral aide 

 (as in Fig. 123). The chest and ventral walls, with the live A , have been 

 removed; 3, outer nasal process ; 4, upper jaw ; 5, lower jaw ; z, tongue ; v, 

 right, v', left ventricle of heart ; 0', left auricle ; b, origin of the aorta ; 

 b' b" b"' y first, second, third arterial arches ; c, c', c", hollow veins (vena 

 cava) ; ae, lungs (y, arteries of lungs) ; e, stomach ; w, primitive kidneys ; 

 (y, left yelk- veins ; s, vena porta ; a, right yelk-artery ; n, navel-artery ; 

 m, navel-vein) ; x, yelk-duct ; i, large intestine ; 8, tail ; 9, front limb ; 

 9', hind limb. (After Coste.) 



ward, on the yet open chest. The curvature presently 

 becomes so great that the tail almost touches the forehead 

 (Fig. 122, V. ; Fig. 137). Three or four distinct curves of the 

 arched dorsal side are now distinguishable ; a skull-curve or 

 "front head-curve" near the second brain-bladder, a neck- 

 curve or " hind head-curve " at the beginning of the spinal 

 marrow, and a tail-curve at the hind end of the body. This 

 marked curvature is shared by Man with the three 

 higher classes of Vertebrates (the Amnion-animals), while 

 in the lower classes it is either much less pronounced, or 

 altogether absent. In this stage, when four weeks old, Man 

 has a true tail, double the length of the legs. The rudi- 

 ments of the limbs are now plainly marked : four entirely 

 simple buds in the form of roundish plates, two fore limbs 

 and two hind limbs, the former being a little larger than 

 the latter. 104 



On opening the human embryo of the age of one month 

 (Fig. 123), we find the intestinal canal already formed in the 

 body-cavity, and that it is nearly completely separated 

 from the germ-vesicle. The mouth-opening and anus 

 already exist. The cavity of the mouth is, however, not 

 yet separated from that of the nose, nor is the face in 

 general yet formed. The heart, on the other hand, already 

 shows all the four compartments ; it is very large, filling 



