392 
MESSRS. F. M. BALFOUR AND W. N. PARKER ON THE 
The changes which have taken place by this stage with reference to the haemal 
arches of the tail are not very considerable. 
In the case of a few more vertebrae the haemal processes have united into an arch, 
and the spinous processes of the arches in the region of the caudal fin have grown 
considerably in length. A more important change is perhaps the commencement of a 
segmentation of the distal parts of the haemal arches from the proximal. This process 
has not, however, as yet resulted in a complete separation of the two, such as we find 
In the adult. 
If the haemal processes are traced forwards (Plate 29, figs. 75 and 76) from the 
anterior segment where they meet ventrally, it will be found that each haemal process 
consists of a basal portion, adjoining the notochord, and a peripheral portion. These 
two parts are completely continuous, but the line of a future separation is indicated 
by the structure of the cartilage, though not shown in our figures. As the true body 
cavity of the trunk replaces the obliterated body cavity of the caudal region, no break 
of continuity will be found in the structure of the haemal processes (Plates 28 and 29, 
figs. 73 and 74), but while the basal portions grow somewhat larger, the peripheral 
portions gradually elongate and take the form of delicate rods of cartilage extending 
ventralwards, on each side of the body cavity, immediately outside the peritoneal 
membrane, and along the lines of insertion of the intermuscular septa. These rods 
obviously become the ribs of the adult. 
As one travels forwards the ribs become continually longer and more important, 
and though they are at this stage united with the haemal processes in every part of 
the trunk, yet they are much more completely separated from these processes in front 
than behind (Plate 28, fig. 72). 
In front (Plate 28, fig. 72), each rib (rb.), after continuing its ventral course for 
some distance, immediately outside the peritoneal membrane, turns outwards, and 
passes along one of the intermuscular septa till it reaches the epidermis. This 
feature in the position of the ribs is, as has been already pointed out in the ana¬ 
tomical part of this section, characteristic of all the ribs of the adult. 
It is unfortunate that we have had no specimens showing the ribs at an earlier 
stage of development; but it appears hardly open to doubt that the ribs are originally 
continuous with the hcemal processes, and that the indications of a separation between 
those two parts at this stage are not due to a secondary fusion, but to a commencing 
segmentation. 
It further appears, as Muller, Gegenbaur and others have stated, that the ribs 
and haemal processes of the tail are serially homologous structures; but that the 
view maintained by Gotte in his very valuable memoirs on the Vertebrate skeleton is 
also correct to the effect that the hcemal arches of the tail are homologous throughout the 
series of Fishes. 
believe only arisen from the fact that Gegenbaur had not the young larvae to work with by which alone 
this question could be settled 
