20 MAMMARY APPARATUS OF THE MAMMALIA 



lateral region, takes place in a medioventral 

 direction, so that they finally come to lie on 

 either side close to the line o£ origin of the 

 amniotic folds (Figs. 7a and 7b, ma). Pro- 

 cesses of growth cause the epidermal thicken- 

 ings to assume a somewhat lens-shaped form, 

 and to project above the level of the neigh- 

 bouring skin (Fig. 7c). This condition, how- 

 ever, lasts but a short time, for they soon 



Fig. 7b. — Echidna : Semon's Embryo No. 5. Enlarged 

 View of Ventral Part of Preceding Section, 



SHOWING THE PrIMARY-PrIMORDIA. 



begin to flatten (Fig. 7d). Thus towards the 

 end of the embryonic period, they form skin 

 areas only slightly thickened and oval in shape, 

 the length being not much greater than the 

 breadth. 



In consequence of these transformations, the 

 primary-primordia, in the case of young just 

 hatched, appear as very insignificant and hardly 

 recognizable structures. At this stage, only the 



