290 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



continuous row of lateral fins. If this view is right, then the 

 fore and hind limbs represent highly specialized fin-rays. 



A section shows each bud to be composed of undifferentiated 

 mesoblast, with a covering of epiblast (Figs. Ill A p. 139, and 

 237). It represents in structure a process of the undifferen- 

 tiated somatopleure or body- wall. Processes grow into the limb 

 bud from the muscle plate of each segment which is situated 

 opposite the origin of the bud. Each corresponding segment of 

 the spinal cord also sends to the limb bud a nerve process. At 

 least seven body segments contribute to the formation of each 

 limb. 



Changes in External Conformation. — In the 4 th week (Fig. 

 234) the limb buds are unsegmented ; in the 5 th a constriction 



, forearm 



basisofarm f rearm \ flm 



rad. border hand . hi 



ulnar border 



4th week ^th week 7th week 



9th week 



Fio. 234.— Development of the Upper Limb. (After His.) 



marks the hand off; the position of the elbow being indicated in 

 the same week. In the 7 th week the fingers appear as thicken- 

 ings in the webbed hand, the middle digit being indicated first. 

 They become free at the end of the second month ; occasionally 



bash of leg foot leg 

 tib. border. 



fib. border 



4th week 



caudd 



7th week 



5th week 7th week V 



Fig. 235. — Development of the Lower Limb. 



9th week 



they remain attached, the child being born with its fingers in 

 a syndactylous condition. The shoulder remains buried in the 

 body-wall ; the skeletal structures of the shoulder are the first 

 to appear. The corresponding parts of the lower limb appear 

 in the same order as in the arm. 



