STERNUM git 
calcareous matter appear in the body of the Sternum, at the base 
and in the anterior part of the keel ; but the whole does not become 
bone until about the 20th week, and even then the posterior rim 
may yet remain unossified. In the Chick an unpaired /ophosteon 
in the anterior basal portion of the keel and a pair of metostea 
appear a few days before emerging from the shell; and on 
the day of hatching a pair of pro-ostea is added, all these five 
centres extending so as to coalesce 
about 9 days later. On the 18th 
day half of the keel is ossified, 
after which follows the spina; but 
ossification is not complete before 
the bird is 5 or 6 months old. A 
comparison of a Duck’s Breastbone 
with that of a Fowl shews at a 
glance that in the latter the 
consolidation is much less, more 
than two-thirds of it being formed 
by the metasternum and the pos- 
terior lateral as well as the oblique 
processes—all of them persisting 
as outgrowths and being connected 
only by non- cartilaginous mem- 
branaceous tissue. In the macer- 
ated skeleton the spaces between 
these outgrowths appear as deep 
** notches ee or, if distally closed STERNUM OF A Younc Fowt. 
by bone or cartilage, as fenestrx. F.c. coracoidal facet ; K. Keel; Sp.e. and Sp.i. 
Moreover, in many Birds an addi- ae and interna (other letters 
tional process appears between 
the metasternum and the posterior lateral processes: the presence 
of such a processus intermedius divides each posterior “ notch” into 
two, and when a processus obliquus is wanting it is often hard to 
determine whether there is such an intermediate process. These 
posterior “notches” and fenestre have for many years been used by 
the hunter for neat ‘ characters,” and undue value has been attri- 
buted to them, notwithstanding that authorities such as Parker and 
Prof. Selenka have insisted on the far greater taxonomic importance 
of the configuration of the anterior portion of the Sternum.t The 
variable mode of connexion of the Fwrcula with the keel of the 
Sternum has already been dealt with (SKELETON, p. 858). 
The spina sternt often consists of an inner (dorsal) and an outer 
(ventral) portion; but sometimes they are confluent, or one of 
them may be absent. The shape of the anterior free margin is 
1 This view has also been strongly urged in Phi7. Trans, 1869, p. 337, as 
well as by Prof. Fiirbringer. 
