404 SCIENCE PROGRESS 



these mammary lines in 90 per cent, of the observed cases. ^ 

 It is a curious circumstance that the abnormal presence of 

 these structures is more frequent in men than in women ; 

 this fact alone would lead one to surmise that mammary glands 

 were originally derived from structures which were at least 

 as equally developed in the male as in the female. It is also 

 noteworthy that the percentage of supernumerary mammae is 

 higher than would naturally be suspected, for Bardeleben - 

 found that out of nearly 3,000 recruits as many as 23'3 per cent, 

 possessed additional teats. In many species of mammals the 

 number of the mammae is indeed very inconstant within the 

 limits of the species. 



The mammary glands are usually regarded as modified 

 sebaceous glands, but this opinion does not go far towards 

 solving the question of their origin from a phylogenetic point of 

 view. At this juncture embryology comes to our aid and 

 throws unsuspected light upon the problem. Thus O. Schultze^ 

 discovered that in the young embryos of several mammals (pig, 

 rabbit, etc.) a dorso-lateral ridge is formed on each side, and that 

 the mammae and nipples are eventually developed at points upon 

 this ridge. By subsequent changes of growth these two dorso- 

 lateral mammary lines are finally brought down into the ventro- 

 lateral position that is permanent in the adult. Now the 

 dorso-lateral position of these lines in the embryo exactly 

 corresponds to the position of one of the lateral lines in 

 Amphibia;^ it seems to me therefore within the range of 

 probability that by the very common occurrence of a change 

 in function the lateral line has become transformed into the 

 mammary line in the course of evolution from amphibians 

 to mammals, and has subsequently become differentiated into 

 separate mammae. The glands in both cases are similarly 

 situated and in both cases arise in the Malpighian stratum of 

 the derma. 



' Leichtenstern, Ueberziihlige Briiste, Virch. Arch. f. path. Anat. ^ Phys. 1878, 

 Ixxiii. 222. 



^ Quoted by Bateson, Materials for the Study of Variation., p. 183. 

 London, 1894. 



' Vcrh. d. phys. med. Ges. su IViirzburg^ xxv. 1893, p. 171. 



* Additional force is lent to my theory by the fact that there are several 

 lateral lines in Proteus and all Amphibian larvce (fig. i), not only a dorso- 

 lateral but also an axillary-inguinal, exactly corresponding in position to the 

 mammary line. 



