784 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



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 commence- 

 ment of a segmentation of the distal parts of the haemal arches 

 from the proximal. This process has not, however, as yet re- 

 sulted in a complete separation of the two, such as we find in 

 the adult. 



If the haemal processes are traced forwards (Plate 42, 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 shewn 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 41 and 42, figs. 73 and 74), but 

 while the basal portions graw 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 41, fig. 72). 



In front (Plate 41, fig. 72), each rib (rb.), after continuing its 

 ventral course for some distance, immediately outside the peri- 

 toneal 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 anatomical part of this section, characteristic of all the ribs 

 of the adult. 



It is unfortunate that we have had no specimens shewing the 

 ribs at an earlier stage of development ; but it appears hardly 

 open to doubt that the ribs are originally continuous ivith the 

 hcstnal processes, and that the indications of a separation between 



