The development of tlie urinofjenital organs of the lamprey. 21 



pleure is rounded aud contains a cavity which does not communicate, 

 except by means of the preceding section with the nephrocœle. 

 Sections 12 to 15 show that the projection d has the form of a tubule 

 extending backwards and gradually freeing itself from the underlying 

 mesoderm which no longer contains a trace of the body cavity. In 

 Fig. 16, from a section passing through the middle of the next 

 posterior segment, the nephrocœle {cœ) reappears and the tubule d is 

 seen lying in close contact with the somatopleure. In Fig. 17 the 

 tubule is fused with the somatopleure and essentially the same con- 

 dition as in Fig. 10 is repeated. The cavity of the projection again 

 communicates with the nephrocœle at this point. The sections re- 

 presented in Figs. 18 — 22 repeat the conditions seen in sections 

 12—16 and show that the duct again runs back some distance inde- 

 pedent of the underlying somatopleure. In Figs. 22 and 23 the 

 duct again fuses with the somatopleure. In the latter section its 

 lumen is lost. In Fig. 24, representing a section through the middle 

 of another segment, the nephroca^le reappears between the somato- 

 and splanchnopleure and the tubule-like projection is again repeated. 

 The sections behind this show the duct free again for a short distance. 

 The conditions here described for three successive segments were 

 traced through the next four segments to near the point at which the 

 mesoderm was no longer segmented. 



Leaving aside all unessential details of size and time of deve- 

 lopment, it is obvious that the pronephric duct for at least seven 

 segments behind the pronephros is formed in essentially the same 

 manner as the pronephros. It consists of a number of segmental 

 tubular diverticula of the somatic layer close to the myotome. Tur- 

 ning back to Fig. 25, a frontal section through the corresponding 

 portion of a younger embryo, it is obvious that the duct must arise 

 in the segmented region of the mesoderm (between y and z in the 

 Fig.). The duct thus arises in situ and does not grow back independent 

 of the underlying mesoderm. Nor does it ever exhibit any appearances 

 which might be interpreted either as a connection with the ectoderm 

 or as a derivation from that layer. 



In stage 4 the segmentation of the mesoderm has extended back 

 to the tip of the posterior yolk-laden portion of the pistol-shaped 

 embryo. The two pronephric ducts in the anterior two thirds of 

 the body are very distinct. They have freed themselves from the 

 underlying mesoderm and have acquired a continuous lumen. Anteriorly 

 they pass without interruption into the collecting ducts of the pro- 



