128 MAMMARY APPARATUS OF THE MAMMALIA 



separate from the larger ones jo and a, and gradu- 

 ally shift ventrallj. But whereas p and a at 

 once develop into typical knob-shaped primordia, 

 77 and a behave quite differently. These, 

 in fact, become elongated epithelial cylinders 

 growing into the cutis, and gradually assume 

 the form of well-marked hair primordia, con- 

 nected with special nerves and bloodvessels. 



ai 



Fig. 45d. — Nipple Development of "Sciueus vul- 

 garis." Embryo No. 6. Transverse Section 

 (Diagrammatic) through the Primordia. 



a^ and a, First abdominal and smaller abdominal primary nipple 

 primordia ; big, bloodvessel ; n, nerve. 



Thus each of these primordia produces a strong 

 vibrissa (Fig. 46). Such vibrissge are of course 

 well known on the head and the extremities, 

 but here in the squirrel we find them also on 

 the abdomen to the number of at least four 

 in correspondence with their origin from the 

 two anterior pairs of nipple primordia. Further 

 it is extremely interesting that the accessory 



