194 MEBJSTIC VARIATION. [part I. 



six or seven, are determined together. How or at what stage such 

 determination is made, there is no direct evidence to shew. 



The various arrangements seen suggest then that the relative 

 positions occupied by the mammas depend partly on the number 

 that are present, and that the position of each mamma is to some 

 extent dependent on the position of other mamma?, especially of its 

 neighbours. In this connexion the cases F and L are interesting 

 ones (Fig. 34). In L for example, the 1st on the left is at a higher 

 level than the 1st on the right. It is succeeded by a rudimentary 

 2nd having none on the same level on the other side. The left 

 3rd is behind the right 2nd, but posterior to this point the nipples 

 are approximately paired. These appearances suggest that the dis- 

 placement of the 1st and 3rd on the left are in some way connected 

 with the presence of the rudimentary left 2nd. Similarly in F the 

 left 3rd and 5th are spaced out for the rudimentary 4th. From its 

 position and small size it might fairly be supposed that this is a 

 " supernumerary " organ, for at all events it is visibly different from 

 the others : but in the case of seven on each side in pairs, no one 

 mamma rather than another can be pointed out as obviously 

 supernumerary when compared with a similar series of six. It 

 seems therefore that of the factors determining the relative posi- 

 tions of the mammas along the mammary lines, the number of the 

 mammae is one, and that the positions of the mamma? are in some 

 way and to a limited extent correlated with each other. That there 

 are other factors at work, also, is sufficiently shown by the exist- 

 ence of cases of apparently utter irregularity. 



In seeking to go beyond this and inquire as to the way in 

 which this correlation is brought about there is, in the present 

 state of knowledge of the mechanics of Division, not much to be 

 gained. Reference may be made to recent observations published 

 in abstract by O. Schultze 1 . According to him there is in young 

 embryos of several mammals (Pig 1*5 cm. long; Rabbit 13 — 14 

 days, &c.) a ridge running along the dorso-lateral aspect on each 

 side and at points upon this the mamma? and nipples are even- 

 tually formed. (The formation of the true nipples is preceded by 

 the raising of the epidermis into small elevations, " primitive 

 teats," which afterwards disappear.) The two mammary lines are 

 by subsequent changes and growth of the body brought into the 

 ventro-lateral position. The question of the position of the mamma? 

 therefore resolves itself into this : what determines the positions 

 at which mammary centres, to borrow the word used in the case 

 of bone, are to be formed on the mammary lines ? In a subse- 

 quent place I shall contend that the facts given are only intelli- 

 gible on the view that the forces determining the points of growth 

 of mamma? are compounded into one system of forces. But to the 

 question what are these forces there is no answer. 



1 O. Schultze, Anat. Anz., 1892, vn. p. 265, since published in full (Verh. d. 

 j)hys.-med. Ges. zu WUrzburg, xxvi. 1893, p. 171, Pis.). 



