632 EMBRYOLOGY. 



of the body. The first segmentation of the vertebrate body affects 

 the body-sacs and the musculature arising from them. The forma- 

 tion of the primitive segments is only remotely and indirectly 

 connected with the development and segmentation of the vertebral 

 column. It is only after muscle-segments have existed for a long 

 time that, at a comparatively late stage of development, the funda- 

 ments of a segmented vertebral column are established. But these 

 arise, by histological metamorphosis, from an unssgmented con- 

 nective-tissue matrix, in consequence of the appearance of a process 

 of chondrification. 



' All the conditions here only briefly touched upon are of far- 

 reaching significance for the question of the relation of the head- and 

 trunk-skeletons to each other. For, as Gegenbaur rightly points 

 out, since the establishment of his segmental theory " the vertebral 

 theory of the skull has become more and more a problem of the 

 phylogenesis of the whole head." 



I desire to give briefly and conneotedlj- my own views upon this 

 subject : — 



Theory concerning the Relation of the Head and its Skeleton 

 to the Skeleton of the Trunk. 



The segmentation of the vertebrate body begins with the walls of 

 the primary body-sacs, the dorsal portion of which, abutting upon 

 the chorda and neural tube, is divided by the formation of folds into 

 successive compartments, the primitive segments. 



Inasmuch as the voluntary musculature is developed from the 

 walls of the primitive segments, it is the first system of organs in 

 Vertebrates to be segmented. 



The myomeric condition — " myomerism " — ^is the direct cause of a 

 segmental arrangement of the peripheral nerve-tracts, for the motor 

 nerves pertaining to a segment unite to form an anterior [ventral] 

 root as they emerge from the spinal cord, and in the same manner 

 the sensory nerves which come from a corresponding part of the skin 

 together constitute a sensory root. 



At a. time when the segmentation of the musculature and of the 

 peripheral nerve-tracts has already been effected, the skeleton is 

 still unsegmented ; for it is represented by the chorda dorsalis alone. 

 The soft mesenchyme, which envelops the chorda and the neural 

 tube, and which becomes the matrix of the subsequently formed 

 segmented axial skeleton, is still a continuous mass of cells, filling in 

 the spaces between these organs. 



