DEVELOPMENT OF ELASMOBRANCH FISHES. 127 



formed, their centres corresponded, not with the centres of 

 the muscle-plates, but with the inter-muscular septa. 



These considerations fully explain the secondary segmen- 

 tation of the vertebrae by which they become opposite the 

 inter-muscular septa. On the other hand, the primary seg- 

 mentation is clearly a remnant of the time when no ver- 

 tebral bodies were present, and has no greater morphological 

 significance than the fact that the cells to form the unseg- 

 mented investment of the notochord were derived from the 

 segmented muscle-plates, and only secondarily became fused 

 into a continuous tube. 



The Urino-genital System. 



The first traces of the urinary system become visible at 

 about the time of the appearance of the third visceral cleft. At 

 about this period the somatopleure and splanchnopleure become 

 more or less fused together at the level of the dorsal aorta, and 

 thus, as has been already mentioned, each of the original plates 

 of mesoblast becomes divided into a vertebral plate and lateral 

 plate (PI. X. fig. 6). The mass of cells resulting from this fusion 

 corresponds with Waldeyer's intermediate cell-mass in the Fowl. 



At about the level of fifth protovertebra the first trace of 

 the urinary system appears. 



From the intermediate cell-mass a solid knob grows outwards 

 towards the epiblast (woodcut, fig. 4, pd). This knob consists at 

 first of 20 — 30 cells, which agree in character with the neigh- 

 bourinoj cells of the intermediate cell-mass, and are at this 

 period rounded. It is mainly, if not enthely, derived from the 

 somatic layer of the mesoblast. 



From this knob there grows backwards a solid rod of cells 

 which keeps in very close contact with the epiblast, and 

 rapidly diminishes in size towards its posterior extremity. Its 

 hindermost part consists in section of at most one or two cells. 

 It keeps so close to the epiblast that it might be supposed to 

 be derived from that layer were it not for the sections shewing 

 its orio^in from the knob above mentioned. We have in this 

 rod the commencement of what I have elsewhere^ called the 

 segmental duct. 



1 Urinogenital Organs of Vertebrates, Jonrn. of Anot. and Phys. Vol. x. 



