CLELAND ON RIDS AND TRANSVERSE PROCESSES. 
125 
they are formed, and which are portions of the middle layer of the 
embryo; and the observations of Bemak have shown us that from 
each dorsal plate there is a nervous, an osseous and a muscular ele¬ 
ment developed, which all, stretching out in the plane of the primary 
layers, tend to complete their circles. But the laminae of the ver¬ 
tebrae are at right angles to the main direction of the dorsal plates 
and the layers of the embryo; and even as the elevations of blastema 
in which they are developed are processes from the middle layer of 
the embryo, so do the laminae diverge from the main circle of the 
skeleton, and in that respect they are to be compared with the 
various processes directed outwards to the skin in fishes, and with 
the epicostal bones in birds. 
Owing, however, to the embryo being only a bud from the 
germinal membrane, not enclosing the complete sphere of the ovum, 
the rings in which its parts lie do not pre-exist, but are gradually 
formed by growth from one side and subsequent closure. From the 
very first, therefore, symmetry between the dorsal and ventral aspects 
is impossible. Development proceeds from the dorsal to the ventral 
aspect, because the dorsum is, in the first instance, the centre; while 
that which is to become the ventral mesial line is the circumference 
of the embryo. Hence the importance in position of the chorda 
dorsalis. We can freely admit that the chorda dorsalis is the centre 
of development of the skeleton, and yet recognise that the skeleton 
and other systems primarily circle round the alimentary tube. 
From the foregoing argument, it follows that ribs embracing the 
visceral cavity correspond, and that transverse processes supporting 
ribs correspond. They are members of the primary circle, and the 
exact place and manner in which they are articulated or ossified to 
the vertebral column depends on details of later development. 
There is thus no difficulty in recognising the correspondence between 
the ordinary transverse processes of fishes and those of mammals. 
But it will be said that the point of articulation of the head of 
the rib is, in that case, the part in the mammal; bird, or reptile which 
corresponds with the transverse process in fishes. On that subject 
it may be noted that while we have abundance of instances of mam¬ 
malian, ornithic and reptilian ribs supported by transverse processes 
only, we have no instance of the rib failing to articulate with the 
transverse process when the latter is present. It was previously 
pointed out that the floating ribs in man, generally supposed to be an 
exception to this, do really articulate with the transverse processes, 
though not by means of synovial surfaces, and that, in cases of 
thirteen ribs, the additional rib is attached to the transverse process 
of the first lumbar vertebra. This accords so far with Bathke’s 
statement, that “ where the transverse process occurs, the rib is, in 
all cases, at first united only with it; sometimes, however, a process 
grows out from the rib (the so-called head with its neck), by which 
it becomes immediately attached to the body of the vertebra itself, 
