86 WILLIAM MORTON WHEELER, 



culum as in the more anterior section represented in Fig. 37. This is 

 from a slightly older embryo than that of Fig. 38. 



Fig. 40. Stage 4. Sagittal section of the right pronephros. The 

 pronephric tubules have lengthened and have become more attenuated. 

 The collecting duct (cd) uniting the five tubules is clearly seen. The 

 second and third pronephric funnels are acquiring cilia. 



Fig. 41. Stage 4. A combination of two successive sagittal 

 sections representig the pronephros and anterior end of the pronephric 

 duct. In this embryo the collecting duct was distended with liquid. 

 The arrangement of the five tubules is essentially the same as that of 

 Fig. 40. 



Fig. 42 — 43. Stage 4. Two successive cross-sections to show 

 the openings of the pronephric duct (d) into the posterior portion of 

 the intestine (int). Both sections show the huge yolk-laden repro- 

 ductive cells (gon) in the mesoderm lateral to the duct. 



Plate 5. 



Fig. 44. Stage 10. Cross-section through the left pronephros. 

 The glomus is distinctly formed and supplied by a vessel from the 

 aorta (gla). The tubules of the pronephros (pr07l) have become contorted, 

 their funnels ciliated {pron.f). 



Fig. 45. Stage 6. Cross-section through the posterior portion 

 of the body, showing that the reproductive cells {gon) from either side 

 move from their lateral position towards the median line under the 

 pronephric duct (d) (conf. Fig. 42 and 43). 



Fig. 46. Stage 12. Cross-section through the middle of the 

 bod}^ The two rudiments of the reproductive organ have met and 

 fused in the middle line below the aorta. 



Fig. 47. Stage 11. Parts of a cross-section through the middle 

 of a young embryo showing the reproductive rudiment just after the 

 two halves have fused. The intestinal and reproductive cells still 

 contain yolk granules and the spaces in the tissues on either side be- 

 tween the aorta (ao) and the cardinal vein (co) are filled with a sub- 

 stance resembling liquified yolk. 



Fig. 48. Cross-section of pronephros of an Ammocoetes 9,5 cm 

 long (stage 20). In this figure two sections have been combined, one 

 containing the glomus and part of its artery. 



Fig. 49 — 51. Reconstructions of the pronephros in stages 4, 5 

 and 7. The first funnel is obliterated. The convolutions of the tubules 

 of Fig. 51 are not all in the plane of the paper as represented. 



Plate 6. 



Fig. 52 — 57. Stage 16, (Petroniyzon marinus dorsatus, 15 mm 

 long) Various portions of the nephric fold containing the pronephric 



