388 
MESSRS. F. M. BALFOUR ART) W. N. PARKER ON THE 
As regards the behaviour of the ribs in the transitional region between the trunk 
and the tail, we cannot do better than translate the description given bv Gegenbaur. 
of this region (No. 6, p. 411)“ Up to the 34th vertebra the ribs borne by the late¬ 
rally and posteriorly directed processes present nothing remarkable, though they have 
gradually become shorter. The ribs of the 35th vertebra exhibit a slight curvature 
outwards of their free ends, a peculiarity still more marked in the 36th. The last 
named pair of ribs converge somewhat in their descent backwards so that both ribs 
decidedly approach before bending outwards. The 37th vertebra is no longer pro¬ 
vided with freely terminating ribs, but on the contrary, the same pair of processes 
which in front was provided with ribs, bears a short forked process as the hsemal arch. 
The tivo, up to this point separated ribs, have here formed a haemal arch by the fusion 
of their lower ends, which arch is movable just like the ribs, and, like them, is attached 
to the vertebral column 
In the region of the tail fin the hsemal arches supporting the caudal fin rays are 
very much enlarged. 
Part II.— Development of the vertebral column and ribs. 
The first development and early histological changes of the notochord have already 
been given, and we may take up the history of the vertebral column at a period when 
the notochord forms a large circular rod, whose cells are already highly vacuolated, 
while the septa between the vacuoles form a delicate wide-meshed reticulum. Sur¬ 
rounding the notochord is the usual cuticular sheath, which is still thin. 
The first indications of the future vertebral column are to be found in the formation 
of a distinct mesoblastic investment of the notochord. On the dorsal aspect of the 
notochord, the mesoblast forms two ridges, one on each side, which are prolonged 
upwards so as to meet above the neural canal, for which they form a kind of sheath. 
On the ventral side of the notochord there are also two ridges, which are, however, 
except on the tail, much less prominent than the dorsal ridges. 
The changes which next ensue are practically identical with those which take place 
in Teleostei. Around the cuticular sheath of the notochord there is formed an elastic 
membrane—the membrana elastica externa. At the same time the basal parts of the 
dorsal, or as we may perhaps more conveniently call them, the neural ridges of the 
notochord become enlarged at each intermuscular septum, and the tissue of these en¬ 
largements soon becomes converted into cartilage, thus forming a series of independent 
paired neural processes riding on the membrana elastica externa surrounding the 
notochord, and extending about two-thirds of the way up the sides of the medullary 
cord. They are shown in transverse section in Plate 28, fig. 67 (n.a.), and in a side 
view in fig. 68 (n.a.). 
Simultaneously with the neural arches, the hsemal arches also become established, 
and arise by the formation of similar enlargements of the ventral or hsemal ridges. 
In the trunk they are very small, but in the region of the tail their condition is very 
