164. 



THE LIMBS. 



Each limb consists of a part which is sunk in the substance of the lateral ridge, 

 and in which the thoracic or pelvic girdle becomes developed, and of a free or 



-sate 



Fig. 194. TRANSVERSE SECTION THROUGH AN ANTERIOR PART OF THE TRUNK OF AN EMBRYO OF 



SCYLLIUM. (BaJfour.j 



gp.c, spinal cord; ,97.7, ganglion of posterior root; ar, anterior root; dn, dorsal ; sp.n, ventral 

 branch of spinal nerve ; mp, muscle plate ; mp', part of muscle plate already converted into muscle ; 

 mp.l, part of muscle plate extending into the limb ; nl, nei-vus lateralis ; ao, aorta ; ch, notochord ; 

 sy'9> sympathetic ganglion ; ca.v, cardinal vein ; sd, segmental duct ; st, segmental tube ; du, duode- 

 num ; hp.d, junction of hepatic duct \vith it; pan, rudiment of pancreas connected with another part 

 of duodenum ; umc, opening of umbilical canal (vitelline duct). 



projecting part (fig. 195). This is at first quite simple, and represents the distal 

 segment of the limb (hand or foot). The other two segments (forearm and leg ; 

 arm and thigh) are successively marked off between it and the girdle by the 

 development of transverse furrows representing the joints (fifth and sixth week). 

 At about the same time four notches appear in the flattened distal extremity, 

 marking off the intervals between the fingers and toes, and the middle segment 

 (fore-arm or leg) begins to be flexed upon the proximal (arm or thigh), the 



the limbs .do not i-eceive such prolongations from the muscle-plates in birds and mammals ; the muscles 

 develop in situ from previously indifferent mesoblast. 



