778 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



peritoneal membrane, but pass outwards along the intermuscular 

 septa till their free ends come into very close proximity with the 

 skin. This peculiarity, which holds good in the adult for all the 

 free ribs, is shewn in one of the anterior ribs of an advanced 

 larva in Plate 41, fig. 72 (rb.}. We are not aware that this has 

 been previously noticed, but it appears to us to be a point not 

 without interest in all questions which concern the homology of 

 rib-like structures occupying different positions in relation to the 

 muscles. Its bearings are fully dealt with in the section of this 

 paper devoted to the consideration of the homologies of the ribs 

 in Fishes. 



As regards the behaviour of the ribs in the transitional region 

 between the trunk and the tail, we cannot do better than trans- 

 late the description given by 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 remark- 

 able, 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 back- 

 wards so that both ribs decidedly approach before bending out- 

 wards. The 37th vertebra is no longer provided 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 haemal arch. The two, 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 haemal 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 vacuo- 

 lated, while the septa between the vacuoles form a delicate 



