THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 627 



fuses with the covering bone. The remainder of that portion of 

 MECKEL'S cartilage which is enclosed in the canal of the lower jaw, 

 from the foramen alveolare onward, is gradually broken down and 

 dissolved ; however, remnants of the cartilage are found even in the 

 new-born infant at the symphysis. 



At first the bony lower jaw is a paired structure, consisting of 

 tooth-bearing halves. These remain in many Mammals as separate 

 bones, being united in a symphysis by means of connective tissue. 

 In Man they are united in the first year after birth into a single 

 piece by the ossification of the intervening tissue. 



A special peculiarity is exhibited by the articular end of the lower 

 jaw, phylogenetically a covering bone. Instead of beginning to be 

 formed, in the manner of the anterior portion, by direct ossification 

 of the connective-tissue foundation, there first arises here a carti- 

 laginous tissue consisting of large vesicular cells and soft intercelluar 

 substance, which is gradually converted into bone. This presents 

 a certain similarity to the development of the primordial bones. 

 But that the resemblance is only superficial is shown by the differ- 

 ence in the structure of the articulation, to which I shall ^return in 

 a subsequent section. 



(c) Concerning the Relation of the Head-Skeleton to the 

 Trunk-Skeleton. 



In different sections of this text-book in discussing the primitive 

 segments, the nervous system, and especially now in the discussion 

 of the axial skeleton reference has been made to many points 

 of agreement that have been recognised between the structural 

 conditions of the head and those of the trunk. In a critical com- 

 parison of these two regions of the body there arise many important 

 questions which have for several decades engaged the attention of 

 the best morphologists. It may therefore be well here, after having 

 given the pertinent facts, to take up these questions more particularly, 

 and determine the relation which head and trunk-, and especially that 

 which head-skeleton and trunk-skeleton, sustain to each other. 



Before I elucidate the present state of the question, I will give a 

 brief survey of the history of these researches, which have been 

 grouped together under the name 



" The Vertebral Theory of the Skull" 

 The relation which the anterior and posterior parts of the skeleton 



