CHAP. IX.] TEETH : PRELIMINARY. 197 



In speaking of the relation of the series of the upper jaw to 

 that of the lower jaw as one of images, it must be remembered 

 that the expression is only very loosely applicable. In particular 

 it should be noticed that though in so far as the lower teeth are a 

 copy of the upper ones the resemblance is one of images, yet the 

 teeth which resemble each other do not usually stand opposite to 

 each other in the bite, but members of the upper series alternate 

 with those of the lower. The incisors, as a rule, however, and the 

 back teeth of a certain number of forms do bite opposite each 

 other, and in them the relation of images is fairly close. 



The importance of the recognition of the relation of images as 

 subsisting between the teeth of the upper and lower jaws will be 

 seen when this case is compared with that of the two sides of the 

 body. For ordinary bilateral symmetry is, as has already been 

 suggested, an expression of the original equality and similarity of 

 the two halves into which the ovum was divided by the first 

 cleavage-plane, or by one of the cleavages shortly succeeding upon 

 this. The fact that the two halves of the body are images of each 

 other is thus both an evidence and a consequence of the fact that 

 the forces dividing the ovum into two similar halves are equal and 

 opposite to each other. The bilateral symmetry of Variation is 

 thus only a special case of this principle. 



In view of the fact that the teeth in the upper and lower jaws 

 may vary simultaneously and similarly, just as the two halves of the 

 body may do, it seems likely that the division of the tissues to form 

 the mouth-slit must be a process in this respect comparable with 

 a cleavage along the future middle line of the body. It is difficult, 

 however, to realize the actual occurrence of such a process of 

 division in the case of the slit forming the original stomodoeum, 

 and this difficulty is increased by the recent observations of 

 SEDGWICK 1 to the effect that in the Elasmobranchs examined by 

 him the mouth-slit first appears as a longitudinal row of pores. If 

 this is so the relation of images must exist in the case of the 

 mouth, not only in respect of the two sides of the slit, but also in 

 respect of the anterior and posterior extensions of the slit. But 

 whatever may be the processes by which the tissues bounding the 

 mouth of a vertebrate come apart from each other,the result is clearly 

 in many cases to produce an anterior series of organs in the upper 

 jaw, related to a posterior series of organs in the lower jaw, much 

 in the same way that the right side of a jaw is related to the left 

 of the same jaw. This relation may appear as has been stated, 

 not only in the normal resemblances between the upper and lower 

 teeth, but also in the fact that similar and simultaneous Variation 

 is possible to them. 



In another respect the Repetition of teeth may differ from that 

 of other Linear Series already considered. In many animals, the 



1 SEDGWICK, A., Quart. Jour. Jlicr. Sci., 1892, p. 570. 



