66 NORTH AMERICAN FAUNA 74 



Fig. 44. Mystacial vibrissae of a fetus (left) 6 months post-implantation, and of a calf 

 (right) about 1 month old. In the fetus (right anterior view), the tips of the slender, 

 unpigmented vibrissae are blunt and frayed, whereas those of the stout, pigmented 

 vibrissae of the calf (sagittal section of snout skin) have been rounded by abrasion. 

 (Photos by F. H. Fay and G. C. Kelley) 



The mystacial vibrissae are the first hairs to appear in the fetus. Some were 

 present in my youngest specimen, 1.5 months post-implantation, in which the 

 primordia of the pelage hair follicles were not yet fully differentiated. The fetal 

 vibrissae are soft, white, and opaque, and more slender and curly than those of 

 calves. In the fetuses, the vibrissae on the upper part of the snout usually were 

 the shortest and most slender; those in the lower lateral quadrants were longest 

 and stoutest. Most of them curved downward and inward, toward the mouth or 

 chin, and the tips were blunt (broken) or frayed (Fig. 44). 



The mystacial vibrissae of newborn calves were like those of the fetuses, but 

 within 3 to 4 weeks after birth they were appreciably stouter, apparently having 

 grown considerably in that time. The root and proximal part of the shaft were 

 heavily pigmented in these older calves, and the tips were worn smooth, rather 

 than frayed or broken (Fig. 44). Maximum basal diameters of the larger vibrissae 

 ranged from 0.9 to 1.2 mm. Some in the upper lateral quadrants of the snout 

 were worn down to the level of the surrounding skin; the rest were about three- 

 fourths as long as those in newborn calves. 



Most mystacial vibrissae present in 1 -year-old animals were nearly twice as 

 stout but about half as long as those of the calves. They were whitish at the tips, 

 grading to dark brown at the base, those in the uppermost rows being palest. The 

 tips were smoothed or beveled, as a result of abrasion, and a large proportion of 

 the vibrissae in the upper third of the snout was worn down to or below the level 

 of the surrounding skin. In progressively older animals, the vibrissae tended to be 



