i) 
feather is uearly fully formed has become firm and resistant, while the 
lower growing part is still soft and plastic and capable of shrinking. Under 
these conditions any temporary lowering of the blood-pressure, as by 
partial withdrawal of the blood, will have to be met by the shrinkage of 
the wall of the tube, and this naturally takes place by wrinkling at the 
weakest places, the night rings within the socket. In the process of growth 
the lower parts of the tube, along with the wrinklings, passes higher and 
higher, and at the same time becomes more horny and resistant. More- 
-over, it would seem that the wrinklings once produced are never 
straightened out by any subsequent restoration of the pressure; indeed 
they become intensified as they grow upwards on account of the normal 
drying-up of the feather as the blood is withdrawn from the middle. 
Good examples of the wrinklings on feathers are sometimes produced 
when a growing feather is drawn from the socket and then allowed to dry. 
‘The plucking of the unripe feather results in a loss of blood from the 
medulla, and the internal pressure being thereby dimished the sheath 
tends to shrivel upon the parts inside; the shrivelling takes place at the 
night rings, and the wrinklings assume the form of those produced 
naturally in barring feathers. 
The deepest constrictions are rarely produced at each successive 
night ring, but, as shown in Figs. 5 and 6, may be about an inch apart, 
which represents four or five days’ growth. Evidently a single collapse 
suffices to meet the variations of the blood pressure within this period, and 
there is no necessity for an indentation at each ring. By the time the 
feather at the wrinkling has grown an inch or so the sheath is becoming 
firm and resistant and not sufficiently plastic to continue to adapt itself 
to the pressure variations, and therefore a new collapse is formed lower 
down where the sheath is weakest. 
How THE WRINKLINGS PRODUCE Bars. 
With the above before us the production of a bar may be understood 
as follows. The wrinkling of the feather-sheath and the indentation of 
the feather substance below always occur while the feather is in unformed 
condition and within the socket. When this takes place the nutritive 
fluids in the region of the wrinklings will in some measure be squeezed 
away, and asa result of this and the compression the parts of the feather 
will not form properly, the barbs will be thinner and the barbules: 
shorter, and a place of weakness and feather deficiency will result, to be 
revealed when the plume opens out. The deeper the wrinkle the greater 
is the compression, the less flue is formed beneath it, and the more 
conspicuous is the bar; so that all degrees in the production of a bar are 
represented, often on the same feather. Some are barely noticeable, while 
others are very evident; moreover any wrinkle rarely passes all round the 
growing feather, and in this we have the explanation of the fact that the 
bars do not often extend completely across the plume. 
It has been supposed that the mere alternation of the day and night 
growth would suffice to produce conspicuous bars in birds, but so far 
as the ostrich is concerned, this is not the case. Jt is only when the 
wrinkling takes place in addition to the weakened night growth that defects 
of importance appear; hence on the growing feather, even before the plume 
opens out, it is quite easy to see whether or not bars are forming. It 
would be difficult to conceive that the difference between the nutrition 
of the growing feather during the day and the nigh. could be so great 
that in one case the barbules would be fully formed while in the other 
they would be absent, whereas all the appearances can be satisfactorily 
explained by the production of the indentations from the partial collapse 
