THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 635* 



arches that the corresponding skeletal axis must likewise have been 

 segmented. They are only an indication of the segmentation of the- 

 region of the body to which they belong. 



That the segmentation of the head which is present in the embryo 

 is more or less obliterated in the adult Vertebrate depends upon 

 two causes. First the primitive segments are only slightly developed, 

 furnishing unimportant muscles, and in part wholly degenerate; 

 secondly the visceral skeleton is subjected to profound metamorphoses. 

 Especially in the higher Vertebrates it experiences such a degenera- 

 tion and metamorphosis, that finally nothing of the original segmental 

 arrangement of its parts (palato-maxillary apparatus, auditory 

 ossicles, hyoid bone) is left. 



B. The Development of the Skeleton of the Extremities. 



A description of the skeleton of the extremities should be preceded 

 by a few words ^ g3 zb ^ 



in regard to the | 



fundaments of the 

 limbs themselves. 

 These at first 

 appear as small 

 elevations [limb- 

 buds] at the sides 

 of the trunk in 

 front and behind 

 (fig. 339). That 

 they belong more 

 to the ventral than 

 to the dorsal sur- 

 face of the body is 

 evident from the 

 fact that they are 

 innervated by the 

 ventral branches 

 of the spinal 

 nerves. 



Moreover, the 

 limbs appear to 



Fig. 339. Very young human embryo of the fourth w eek 4 mm 

 long, neck-rump measurement ; taken from the uterus of a 

 suicide 8 hours after her death, after RABL. 



au, Eye ; ng, nasal pit ; uk, lower jaw ; zb, hyoid arch ; s 3 , s*, third 

 and fourth visceral arches ; h, protrusion of the wall of the 

 trunk caused by the growth of the heart ; us, boundary between 

 two primitive segments ; oe, ue, anterior and posterior limbs. 



belong to a large number of trunk-segments. This is to be inferred both 

 from the method of the distribution of nerves and also from the source 



