No. 557] INHERITANCE OF MAMMJE IN SWINE 259 



individual. Williams particularly contends for this view, 

 claiming that each somite in the original ancestral condi- 

 tion possessed its pair of glands. He also invokes the 

 theory of reversion to account for the definite places in 

 which supernumerary nipples occur. Bateson, on the con- 

 trary, defines the position of the teats as occurring regu- 

 larly on the "mammary lines." These lines diverge 

 toward the axillae and converge toward the inguinal 

 region. He assumes that mammae may appear at any 

 point on these lines and apparently with only slight rela- 

 tionship to the somites. 



Embryology. — Embryologically, Bateson 's idea of the 

 mammary lines seems to be well supported. In the swine 

 embryo of 14r-15 mm. the mammary ridges appear plainly 

 visible in a dorso-lateral position. At about 17 mm. small 

 elevations ("primitive teats") appear on the surface of 

 these ridges. When about 19-20 mm. long, the inter- 

 vening parts of ridges are resorbed, leaving the teats at 

 the point in which they will normally develop. 



This entirely precedes the formation of the true mam- 

 mary tissue. In fact Creighton says that in the kitten 

 the latter may not appear until after birth, although 

 guinea pigs and swine show an earlier development. The 

 factors that govern the transverse division of the lateral 

 mammary tissue into mammae, so far as the writer has 

 been able to discover, are unknown. Most authors have 

 insisted that metamerism has nothing to do with it, but 

 offer little demonstrable in its place. Suffice it to say, 

 however, there is some force that serves to maintain 

 symmetry and regularity of division in the majority of 

 cases. An attempt will be made to show some of its 

 effects in the following study, even though it can not be 

 definitely named and its action described. 



Types of Variation. — The simplest type of variation 

 in the number of mammae consists of one more or one less 

 pair than is the usual number. The writer has assumed 

 the first pair and the inguinal pair constant, because of 

 their definite position. On this assumption, the addition 



