11)4 M ERISTIC VARIATION. [PART I. 



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

 <1. -t. rtninatiou is made, there is no direct evidence to shew. 



The \ari.ius arrangements seen suggest then that the relative 

 positions ormpied l>y the mamma? depend partly on the number 

 ti, : ,t are present, :unf thsit the position of each mamma is to some 

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



rhbours In this connexion the cases F and L are interesting 

 oni - Fii " H. In L for example, the 1st on the left is at a higher 

 |,-v.-l than th.- 1st on the right. It is succeeded by a rudimentary 

 2nd ha\mu r none "ii the same level on the other side. The left 

 :{rd is behind the right "2nd, but posterior to this point the nipples 



approximately paired. These appearances suggest that the dis- 

 placement ofthelsl and .Srd on the left are in some way connected 

 with the | '! sence of the rudimentary left 2nd. Similarly in F the 

 |, .ft :inl and ."ith are -paced out for the rudimentary 4th. From its 

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

 stipernnmerarv "I'gan, for at all events it is visibly different from 

 th,. others: l"it in the case of seven on each side in pairs, no one 

 mamma rat ht-r than another can be pointed out as obviously 

 -npernnmeiarv \\hrn compared with a similar series of six. It 

 seems therefore that ..f the factors determining the relative posi- 

 tion^ nf tin- maniina- :d<>ng the mammary lines, the number of the 

 mamma- i- one, and that the positions of the mamma? are in some 

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

 :ir.' oth'-r factors at work, also, is sufficiently shown by the exist- 

 ence of cases of apparently utter irregularity. 



In sec 'kin 1 .; t" go 1 H -y ond this and inquire as to the way in 

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



Mali- of kliovjed-e of tin- mechanics of Division, HOt lllUcll to be 



:i,-d. Keference may be madi- bo recent observations published 

 in ali-tract 1,\ ( ). Si -iifi.TXK 1 . According to him there is in young 

 embryo- of -, -\.-ral mammals (Pig 1'5 cm. long; Rabbit 13 14 

 dav>, iVc. ) a rid^' 1 running along the dorso-latenil aspect on each 

 -id'f and at points upon this the mamma' and nipples are even- 

 tually fornu'd. (The formation of the true nipples is preceded by 

 tin- raising of the epidermis into small elevations, "primitive 

 beats," \\lii'-li afterwards disappear.) The two mammary lines are 

 li\- -nli-i-i|iient rhaniM-s and growth of the body brought into the 

 \entro-lati-ral position. The i|Urst ion of t lie ^^\\ ion of the mamma' 

 thei-etoi. resolves itself into this: what determines the positions 

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

 of hone, are to lio formed on the inaininarv lilies? In a subse- 

 quent place I shall contend tha the tacts given are only intelli- 

 gible on the \ie\v that the forces dctei-miuing the points of growth 

 of mamma- an- compounded into one -y>tem of forces. But to the 

 i|iie-iioii \\hat are these forces tin-re is no answer. 



1 (). Srnri.T/1 . .lii-it. .!;., Is'.cj, \ n . j,. -jr,.",. since jniblished in full (1'crli. <l. 

 i'.>. :u n'-rJiur;!, xxvi. Is'.i^, p. 171. /'/.--.). 



