No. 2304. REPTILIAN CHARACTERS IN MAMMALS—WORTMAN. 31 



Such an example is seen in tlie dentary bone of the lower jaw, 

 found, as far as I know to the contrary, in all Mammalia; and as this 

 is particularly germane to the subject herein considered, it will be 

 well to mention it fully. For this purpose and in order to bring into 

 stronger relief the elements which enter into a discussion of this sub- 

 ject I quote again at length from the statement of Arthur Thompson 

 in Cunningham's Human Anatomy (p. 141) on the ossification of the 

 human lower jaw, which is as follows: 



The development of the lower jaw is intimately associated with Meckel's cartilage, 

 the cartilaginous bar of the first visceral or mandibular arch. Meckel's cartilages, of 

 which there are two, are connected proximally with the periotic capsule and cranial 

 base. Their distal ends are united in the region of the symphysis. It is in the con- 

 nective tissue overlying the outer surface of this cartilaginous arch that the bulk of 

 the lower jaw is developed. The cartilage itself is not converted into bone, but 

 undergoes resorption, except its anterior extremity, which is stated to undergo ossifi- 

 cation to form the part of the lower jaw lying between the mental foramen and the 

 symphysis. In a third or fourth month fetus the cartilage can be traced from the 

 undersurface of the forepart of the tympanic ring downwards and forwards to reach 

 the jaw, to which it is attached at the opening of the inferior dental canal; from this it 

 may be traced forwards as a narrow strip applied to the inner surface of the mandible, 

 which it sensibly grooves. The proximal end of this furrow remains permanently as 

 the mylohyoid groove. The part of the cartilage between the tympanic ring and the 

 jaw becomes converted into fibrous tissue, and persists in the adult as the internal 

 lateral Ligament of the temporo-maxillary articulation, its proximal end through the 

 Glaserian fissure being continuous with the slender process of the malleus. The part 

 which is applied to the inner surface of the lower jaw disappears. In the tissue over- 

 lying the cartilage, ossification begins by several centers as early as the sixth or seventh 

 week of fetal life, in this respect resembling the clavicle, by which it is alone pre- 

 ceded. The dentary or basal center forms the outer wall and lower border. With 

 this is united the splenial portion, which appears somewhat later, forming the inner 

 table from near the symphysis backwards towards the opening of the inferior dental 

 canal where it terminates in the lingula. By the union of these two parts a groove la 

 formed, which ultimately becomes covered in, and in which the inferior dental nerve 

 and vessels are lodged. As has already been stated, the part of the body between the 

 symphysis and the mental foramen is regarded as directly developed from the fore 

 part of the Meckelian cartilage. As will have been gathered from the above descrip- 

 tion, the upper part of the ramus and its processes have no connection with Meckel's 

 cartilage. The condyle and coronoid process are each developed from a separate center 

 preceded by a cartilaginous matrix. (Italics are mine.) These several centers are all 

 united about the fourth month. 



It may be here noted in regard to his unqualified statement to tlie 

 effect that there is a separate and distmct center of ossification for 

 each of the coronoid and condyloid processes, he is not in accord with 

 ail authorities who have written upon the subject. It is stated by 

 others that while these cartilaginous molds or matrices are present, 

 their actual ossification takes place by an extension of the adjacent 

 membranous layer of the dentary, and that they then undergo absorp- 

 tion. The main facts in connection with the ossification of the lower 

 jaw in man, and, in fact, in all other mammals in which the process is 



