Jeffries.] 
802 
[December 21, 
look through and see the epitrichial layer on the inner side of the 
scale. Below this we come to the horny part of the epidermis 
which is built up of fusiform cells. The outer cells of this layer 
do not always seem to be converted into horn and accordingly 
are stained deep red by carmine, while the lower cells are turned 
into horn and accordingly are not stained by carmine. Below all 
this, at the base of the epidermis, comes a layer of cuboidal cells. 
From this layer, which may be taken as the representative of the 
primitive ectoderm, all the other layers of the epidermis are 
developed, the external being of course divided off first. When 
first divided off these cells very much resemble those of the 
mother layer, but gradually take on the forms peculiar to them- 
selves. About the time of hatching all the external layers down 
to the horn layer dry up and are shed. 
The cutis vera, or derm, is slowly developed during the latter 
part of incubation and of youth, into its final conditions by 
changes, many of which are obscure, and which have no special 
bearing on the subject under consideration. 
Spurs on the wing occur in very different groups of birds, 
always in the vicinity of the carpal joint. They are found in 
Merula didactyloptera (Owen), many of the geese, Didun cuius, 
(Owen), Parra and the like, in Megapodius and in Palamedea 
which has two spurs. These birds are genetically very widely sep- 
arated and it would seem .impossible that the spur can have been 
inherited by all from a common ancestor. The function of the 
spur is given as a weapon. If this be the case it is hard to see 
how it became gradually developed, since till it became large 
enough to project beyond the feathers it could not have been of 
use. 
The claws on the wings of birds are the homologs of those 
on the feet, that is, they are the same structure as those on our 
fingers. Claws were first described by Nitzsch in 1811, who 
pointed out their true homology and considered them proof that 
the wing was a modification of a limb for terrestrial locomotion. 
Nitzsch considered claws of no functional value in the adult, 
as would seem to be the case since they are entirely covered by the 
strong false wing feathers. According to Nitzsch, claws are of 
common occurrence in the diurnal birds of prey and most of the 
