1892,] Zoology. 621 
papille do not push into the deepest but into the lateral portion. In 
this way the dental ridge can continue its growth behind the milk- 
teeth unhindered by the process of separation of these teeth, which 
begins in the fourteenth week. At the same time the ridge shows 
irregular outgrowths and three weeks later these have become more 
evident, and in the region of the incisors a partial fenestration of the 
ridge is begun. 
At the twenty-fourth week the dental ridge, in the region of the 
anterior teeth, has become converted into a sieve-like plate with irreg- 
ular projections; in the molar region it is as yet smooth and unbroken 
and its under margin retains the thickened and undulating character. 
In front of and mesial to these thickenings are the milk teeth, and 
from the side of these milk teeth the papillæ of the permanent teeth 
(the incisors first) encroach upon the thickened margins of the dental 
ridge. The dental ridge grows behind the second milk molar in the 
fourteenth week and at the seventeenth its end is thickened, and from 
this thickening is developed the first permanent molar. 
At the time of birth the ridge extends behind the first molar as a 
short thickened plate which at the sixth month has extended farther 
backward and has become thickened for the second molar which is 
formed as before. In the child of three and a quarter years the con- 
dition behind the second molar is like that in the child at birth, behind 
the first molar. The wisdom tooth arises about the fifth year by a 
lateral ingrowth like that of the other molars. Owing to the extra- 
ordinary adaptability of the dental ridge there is a possibility of a 
fourth molar behind the wisdom tooth and also of a third dentition in 
the anterior portion of the jaws. 
Odontogenesis in the Ungulates.—Julius Laecker, of the 
Veterinär Institut of Dorpat, has just completed a most interesting 
series of studies upon Odontogenesis in the Ungulates. He undertook 
their investigation with the purpose of determining how far the embry- 
ological development of the crowns of the molar teeth repeats the 
palzontological development, and especially to test the theory of 
bunodont origin advocated by Cope and Gaudry as opposed to the 
earlier Riitimeyer-Kowalevsky hypothesis that the primitive molars of 
ungulates were crested or lophodont. His method of research was 
an improvement of that introduced by Klever' upon the molars of 
the horse but his material included not only embryos of the horse, but 
1« Zur Kenntniss der Morphogenese d. Equidengebisses,” Morph, Jahrb. xv, p- 
308, 1889. 
