THE SKELETON 425 
ribs (cervical and thoracic) arise a short distance lateral to the 
vertebral centers, with which they are connected only by the 
intervening membrane, which forms the vertebro-costal liga- 
ments. Chondrification then proceeds distally. 
The cervical ribs chondrify from a single center. The thoracic 
ribs have two centers of chondrification; a proximal one, corre- 
sponding to the vertebrai division of the rib, and a distal one 
corresponding to the sternal division. The lumbar and sacral 
membranous costal processes do not chondrify separately from 
the vertebral bodies; if they persist at all, therefore, they appear 
as processes of the vertebra, and are not considered further. 
In the fowl the atlas does not bear ribs, and in the embryo the primary 
costal processes of this vertebra do not chondrify. The second to the 
fourteenth vertebrae bear short ribs, with capitulum and tuberculum 
bounding the vertebrarterial canal. The fourteenth is the shortest of 
the cervical series. The fifteenth and sixteenth vertebrie bear relatively 
long ribs, but, as these do not reach the sternum, they are classed as 
cervical. The entire embryonic history, however, puts them in the 
same class as the following sternal ribs; on an embryological basis they 
should be classed as incomplete thoracic ribs. They possess no sternal 
division, but the posterior one has an uncinate process like the true tho- 
racal ribs. The following five pairs of ribs (vertebrae 17-21) possess 
vertebral and sternal portions, but the last one fails to reach the sternal 
rib in front of it. 
The vertebral and sternal portions of the true thoracal ribs 
meet at about a right angle in a membranous joint. This bend 
is indicated in the membranous stage of the ribs. 
The membranous ribs growing downwards and backwards 
in the wall of the thorax make a sudden bend forward, and their 
distal extremities fuse (seven and eight days) in a common mem- 
branous expansion (primordium of the sternum), which, however, 
is separated from the corresponding expansion of the opposite 
side by a considerable area of the body-wall. 
The vertebral and sternal portions of the ribs ossify separately ; 
the ossification of the ribs is exclusively perichondral up to at 
least the sixteenth day (cf. Fig. 242). 
The uncinate processes were not formed in any of the embryos 
studied. Apparently they arise as separate membranous ossi- 
fications after hatching. 
The sternum takes its origin from a pair of membranous expan- 
