The MicRosco'PE. 37 



and more space exists between the mesoblastic cells than be- 

 tween the others. 



In Fig. 4, both of these statements are verified, also it will be 

 seen how easily the two layers may be separated artificially. In 

 its incipiency the inferior maxilla arises from two lateral bud- 

 like processes composed of mesoblastic cells nearly surrounded 

 by an epi blast or infant layer. These unite in front, forming the 

 symphasis mentis ; and running through and occupying a cen- 

 tral ])Osition Meckel's cartilage may be found. 



Upon the crest of the gum, in the infant or epiblastic layer 

 which covers it, ma}' first be seen active cell-multiplication which 

 results in a depression or a growing of this layer into the con- 

 nective tissue beneath. This is illustrated in a transverse section. 

 (Fig. 3, x). At certain points along this groove the cell-multipli- 

 cation increases more than at other parts, causing bud-like pro- 

 cesses which extend into the connective tissues beneath forming 

 cords which correspond to the enamel organs of the several tem- 

 porary teeth. 



At the commencement of this infolding of the infant la3'er the 

 band is broad ; from one side a lamina is given off which de- 

 velops into the cord. This develops rapidly, sinking deeper into 

 the mesoblastic or connective tissue, and while at the point of 

 origin it is narrow, at the bottom it broadens, assuming a club 

 or pear-shaped appearance. 



It is made up of a solid ingrowth of cell formed, as just ex- 

 plained, from the lamina, which arose from the band and which 

 in turn originated from the infant epiblast or epithelial layer of 

 the mouth. In a short time the bottom of this cord becomes 

 flattened and finally invaginated by contact with the mesoblastic 

 cells, which now assume new characteristics and become known 

 as the dental papilla, or the future pulp of the tooth. (See Fig. 

 '5, q). The cord and dental papilla now oppose each other in 

 growth, one growing downward into the jaw, the other upward 

 or toward the gum. As a result we have an organ known as the 

 enamel organ, made up from two kinds of cells, epiblastic and 

 mesoblastic, united to accomplish one end, viz., that of forming 

 the tooth. 



This combination, one of the most striking in the animal 

 economy, from this time onward forms a new, independent creat- 

 ure, so to speak, and the result is a product unlike that found 



