DEVELOPMENT OF ELASMOBRANCH FISHES. 261 



terior limb opening into the body cavity and posterior limb, 

 resting on a dilated portion of the segmental duct. The next 

 important change which they undergo consists in a junction 

 being effected between their posterior limbs, and the segmental 

 duct. In the anterior part of the body these junctions appear 

 before the commencement of stage L. A segmental tube at 

 this stage is shewn in longitudinal section on PL XX. fig. 7a, 

 and in transverse section on PL xvii. fig. 2. In the former the 

 actual openings into the body cavity are not visible. In the 

 transverse section only one limb of the A is met with on 

 either side of the section ; the limb opening into the body 

 cavity is seen on the left side, and that opening into the seg- 

 mental duct on the right side. This becomes quite intelligible 

 from a comparison with the longitudinal section which demon- 

 strates that it is clearly not possible to see more than a single 

 limb of the A in any transverse section. 



After the formation of their junctions with the segmental 

 duct, other changes soon take place in the segmental tubes. By 

 the close of stage L four distinct divisions may be noticed in each 

 tube. Firstly, there is the opening into the body cavity, with 

 a somewhat narrow stalk, to which the name segmental tube 

 will be strictly confined in the future, while the whole pro- 

 ducts of the original segmental tube will be spoken of as a 

 segment of the kidney. This narrow stalk opens into a vesicle 

 (PL XVII. fig. 2, and xx. fig. 6), which forms the second divi- 

 sion. From the vesicle proceeds a narrower section forming 

 the third division, which during stage L remains very short, 

 though in later stages it grows with great rapidity. It leads 

 into the fourth division, which constitutes the posterior limb 

 of the A, and has the form of a dilated tube with a narrow 

 opening into the segmental duct. 



The subsequent changes of each segment do not for the 

 most part call for much attention. They consist mainly in the 

 elongation of the third division, and its conversion into a coiled 

 tubulus which then constitutes the main mass of each segment 

 of the kidney. There are, however, two points of some in- 

 terest, viz. : (1) the formation of the Malpighian bodies, and 

 (2) the establishment of the connection between each segmental 

 tube and the tubulus of the preceding segment which was 



