chap, vin.] MAMMAE : COMMENT. 191 



merits of the distances between the nipples on one side in one of 

 these cases were, in inches, 1^, 1, T 7 g, ^, \$ } j£, jf , the rudimentary 

 nipple standing -^ in. from either of its neighbours. In the D 



I o 



O 1 



01 Ol 



os to 



2 °' 



O O 3 



o so 



3 O 



03 30 04 O 3 



o 4 O 3 



30 40 O- 5 „ ,n O * 



O O * o ° 



o 4 



•o 00 _ o o 



O O 0.5 o o 



O O JO 06 



o o 



o o 



8 6 o c 7 7 o o 8 6 o o 7 Coo" 



E F G K L 



Fig. 34. Diagrams of nipples in very young pigs. Letters refer to groups in 

 No. 164. 



group this was found on both sides, but in the E group on one 

 side only, as in the figure (Fig. 34). 



Comment on foregoing evidence. 



On looking at a series of cases like those roughly illustrated in 

 the diagrams, one is tempted to inquire as to the factors which 

 determine the positions of these mammae and nipples. Though 

 such an inquiry must lead to small definite results it may not be 

 unprofitable to point out some deductions which may be made 

 from the facts. I take this opportunity as a good one for illus- 

 trating the position here adopted with respect to the theory of 

 Reversion, and for discussing certain features of the phenomena of 

 Division. 



The mammary glands form an example of a class of Meristic 

 organs which are distributed in series along a body already seg- 

 mented, but whose positions have no obvious coincidence with the 

 fundamental segmentation. In the case of the pig, for instance, it 

 would doubtless be found that the mammae bear more or less 

 definite relations to particular vertebrae, but they are not limited 

 to such positions as the ribs or spinal nerves must be. The seg- 

 mentation of the mammae is thus a segmentation, or serial arrange- 

 ment, superadded upon that of the vertebrae. The question to be 

 considered is, what determines the points at which mammae are to 

 be formed ? 



In the paper to which reference has been made, Williams has 

 contended for the view that each somite bore originally a pair of 

 mammae ; and we may remark that if this were so the problem of 

 the segmentation of the mammae would be the same as that of the 



