HAIR STRUCTURE OF THE MONOTREMATA 483 
hair has been derived from a still finer variety which possessed 
a complete medulla. ‘These isolated medullary cell fragments 
in the shaft are never in a continuous band. Seldom, indeed, 
are they discoverable. Often a close search of the entire length 
of the hair shaft with the highest powers of the microscope is 
necessary before the minute vestiges of the medulla can be de- 
scribed. They are, I think, present in some degree in all wavy 
hairs (figs. 85 and 86). 
In the spiny flattened hairs more of the medulla appears, al- 
though it still exists, in most instances, as streaks of isolated cell 
fragments (fig. 89). At the base of the shaft, in this type of 
hair, and just below the mouth of the follicle, there often occurs 
a large group of medullary cells which resembles the fully formed 
medulla found throughout the shaft of the smaller spines. This 
group of cells pinches out and disappears Just before the emer- 
gence of the hair shaft from the mouth of the follicle (fig. 80). 
Waldeyer observes that in the center of the large, strong spiny 
hair there may occasionally occur large, nodose, pigment masses 
(as he calls them) which perhaps may be regarded as the rudi- 
ments of a medulla. That these masses are medullary cells, [ 
think there is now no doubt. 
At the ectal extremity of the spiny flattened hair of the dor- 
sum there commonly occurs, for the first time in the series, a 
complete medulla (figs. 71 and 72) which persists for a short 
distance down the shaft and then fragments into minute particles 
as the middle of the hair is approached (fig. 73). These fragments 
become progressively smaller and fewer in number in the lower 
portion of the hair (fig. 74), but gather together again into a 
continuous medulla at its base (fig. 75). 
In the smaller spines the medulla is complete, and in the larger 
it reaches its maximum expansion, occupying an important posi- 
tion in the structure of the spine (figs. 94, 95, 96, and 98). 
The cortex of the hairs and spines. ‘The cortex in all the hairs 
and spines is a homogeneous hyaline, horny mass, in which no 
distinct evidences of its fused component cells could with cer- 
tainty be determined. When macerated in a 25 per cent aque- 
ous solution of sodium hydroxide it splits up into a fibrous mass, 
