The development of the urinogeuital organs of the lamprey. §7 



duct (d) and showing successive stages in the formation of the meso- 

 nephric tubules. Tor explanation see text. The sections are drawn 

 under Zeiss ^/j g ^'^•i Oc. 2. 



Pig. 58. Stage 15. (Petromy.son marinus dorsatus, 12 mm long.) 

 Section showing the formation of a mesonephric tubule. 



Fig. 59. Stage 20. (Ammocœtes, 9,5 cm long.) Portion of a 

 sagittal section of the mesonephros showing the elongated glomerulus 

 {mes. gl), numerous mesonephric funnels and tubules and the glandular 

 pronephric duct (d) enclosed in a vascular network (vs). The openings 

 of the tubules into the duct are not cut in this section. 



rig. 60. Stage 20. Sagittal section from the developing meso- 

 nephros of the Ammocœtes represented in Tig. 59, just behind the last 

 completely developed tubules. Clusters of young tubules derived from 

 the peritoneal epithelium are seen converging in groups of three and 

 five to perforate the walls of the pronephric duct [d). 



Pig. 61. Stage 20. Same Ammocœtes. Sagittal section of meso- 

 nephros through the region between the fully developed and the in- 

 completely developed tubules. The line of demarcation is seen to be 

 quite sharp. The clusters of tubules have made somewhat more pro- 

 gress into the duct wall than in Fig. 60. 



Plate 7. 



Fig. 62. Stage 4. Diagrammatic sagittal section showing the 

 relations of the pronephros and its ducts and the reproductive oi'gans 

 to the other portions of the body. 



Fig. 63. Stage 15. (Petromt/^on marinus dorsatus, 12 mm long.) 

 Transverse section of both mesonephric lobes. On either side a meso- 

 nephric funnel with its glomerulus is cut lengthwise. 



Fig. 64. Stage 15. Transverse section of the same Ammocœtes 

 only two segments back of that represented in Fig. 63. The very 

 beginning of a mesonephric tubule as a deeply staining accumulation 

 of peritoneal cells is seen on the left side (mes). On the right the 

 young tubule has grown until it touches the median wall of the pro- 

 nephric duct. 



Fig. 65. Stage 17. ÇPetromyzmi fluviatilis, 22 mm long.) Pro- 

 and mesonephros dissected out and viewed from the mesial surface. 

 Only about half of the length of the pronephric duct is represented. 

 In this Ammocœtes the pronephros had five funnels. 



Fig. 66. Stage 17. A portion of the pronephric duct just 

 behind the last distinctly formed mesonephric tubule (the extreme right 

 end of Fig. 65) highly magnified and seen from the under surface to 

 show the band of peritoneal cells as successive accumulations of 

 nuclei from which the young mesonephric tubules grow inward to meet 

 the pronephric duct. The duct is represented as seen in optical section 

 below the surface of the peritoneum. The band of peritoneal cells lies 

 at first dorsal to the duct, but in the specimen figured soon turns to 

 run along the mesial and ventral surfaces. 



