54 ON THE FOLLICULAR STAGE OF 



for a short time as opaque nodules imbedded in the gum, 

 in the course of the line of adhesion. The existence of 

 germs of canines and superior incisors in the cow and sheep 

 is highly interesting, as it shows how general the law of unity 

 of type is within certain limits. Geoffrey St. Hilaire was the 

 first to announce the existence of tooth-germs in the foetus of 

 the Balama mysticetus, a fact which has been verified by Dr. 

 and Mr. Frederic Kjiox, in whose museum there is a prepara- 

 tion exhibiting the germs under the form of sacs and pulps. 

 Although the germs never arrive at this stage of perfection in 

 the cow and sheep, they are yet distinct enough to indicate 

 their existence ; and I have no doubt that when embryos of 

 other partially or wholly edentulous mammals have been exa- 

 mined, similar results will be obtained. The peculiar manner 

 in which the sac of a ruminant molar, and probably of every 

 other composite tooth, is formed, may be best seen in longi- 

 tudinal or transverse sections of the sac and pulp of the fourth 

 permanent molar of the sheep or cow. The internal surface 

 of the cavity of reserve is seen to end in a fold or folds ; when 

 these meet, they begin to curve towards the papilla, and to 

 enter parallel to one another the cavity or notch which 

 is simultaneously forming in the latter. As soon as the 

 edges of the folds meet, the granular matter denominated 

 enamel-pulp by Hunter (the formation of which was de- 

 scribed in the human embryo, at the last meeting of the 

 Association) begins to be deposited, cementing together the 

 opposing folds, sealing up the new sac, separating it from the 

 rest of the cavity of reserve, filling up the space existing 

 between the pulp and sac, and ultimately assisting in the 

 formation of the depending folds of the latter. 



A distinction must be drawn between those permanent 

 teeth which are developed from the primitive, and those 

 which are developed from the secondary groove. I have been 

 in the habit of dividing the teeth of these animals, the denti- 



