634 EMBRYOLOGY. 



remain in the connective-tissue state. In the head there is developed 

 at once a continuous cartilaginous capsule around the brain-vesicles. 

 The segmentation, which in this region is expressed in other systems of 

 organs, in the formation of primitive segments and in the arrangement 

 of the cranial nerves, does not occur in the corresponding part of the 

 axial skeleton. Never in the course of the development of any 

 Vertebrate has there been observed, as the first fundament of the 

 primordial cranium, a succession of cartilaginous pieces, alternating 

 with connective-tissue discs, and there seems to be no ground for 

 assuming that a condition of this kind existed in earlier times. In- 

 the slight development of the muscles derived from the primitive 

 segments of the head, and in the voluminous condition attained by 

 the brain and sensory organs, are to be discerned, on the contrary, 

 factors which have converted the head, at an early period, into a 

 more rigid portion than the trunk. The cause, which in the trunk 

 has made the segmentation of the axial skeleton necessary, has been 

 wanting in the head. 



During the last few years the opinion has been expressed by 

 a number of persons (ROSENBERG, STOHR, FRORIEP) that in some 

 classes of Vertebrates the occipital region of the primordial cranium 

 is increased by fusion with vertebral fundaments of the neck-region, 

 and thus, as it were, " is constantly advancing caudad." I leave 

 undetermined to what extent this is true. GEGENBAUR combats the 

 interpretation of STOHR, but describes a quite frequently occurring 

 fusion of the cranial capsule with vertebrae in Bony Fishes. One 

 thing only would I point out : the conception of the first unsegmented 

 fundament of the primordial cranium which I have presented is 

 not irreconcilable with the view that subsequently new vertebral 

 segments may be added behind. 



Besides the segmented condition of the vertebrce, a segmentation of 

 the axial skeleton is also expressed in the appearance of ventral arches, 

 which are repeated in regular order from before backwards. On 

 the head they are designated as visceral arches, on the trunk as ribs. 



The position of these skeletal parts also is dependent upon the 

 first segmentation which affects the organisation of Vertebrates. 

 For the ribs are developed between the muscle-segments by a process 

 of chondrification in the connective-tissue plates separating them 

 the intermuscular ligaments ; while the visceral arches are dependent 

 upon the visceral clefts, by which the ventral part of the head-region 

 is divided into a number of successive segments. 



It cannot be concluded from the existence of ribs and visceral 



