The development of the urinogenital organs of the lamprey. 35 



Plate 3. 



Fig. 28. Stage 4. Cross-section through the pronephros (pron. f) 

 and the heart (c). The right collecting duct is enormously distended 

 with liquid. 



Fig. 29. Stage 4. Cross-section through the same region of a 

 normal embryo. There is no blood as get. This section shows the 

 aorta and glomus forming (ao and pron.gl). The sclerotome elements 

 (scI) have made more progress in their dorsal migration between the 

 myotome on the one hand and the chorda and neural tube on the 

 other. 



Fig. 30. Stage 2. Sagittal section through the whole pronephros 

 showing five tubules of the typical relative length. The organ has 

 contracted so that it occupies the space of only three and one half 

 myotomes. 



Fig. 31. Stage 3. Sections through the pronephric tubule showing 

 its relations to the body cavity and peritoneum. 



Fig. 32. Stage 3. Second section beyond that represented in 

 Fig. 31. This section cuts the collecting duct (cd) and grazes the 

 orifice of a pronephric funnel (pron.f). Both in this and the preceding 

 figure the sub-chorda is still distinct and the sclerotome element are 

 beginning to migrate (scl). 



Fig. 33. Stage 3. Cross-section through the posterior end of 

 an embryo. The pronephric duct (d) has not yet separated from the 

 somatic mesoderm and still has no lumen. Lateral to it the large yolk- 

 laden reproductive cells (gon) lie embedded in the mesoderm. 



Fig. 34. Stage 2. Through the same region as that of the pre- 

 ceding section. The pronephric duct has not yet made its appearance 

 and the reproductive cells (gon) are less distinctly separated from the 

 entoderm. 



Fig. 35. Cross- section of pronephric tubules from an Ammoccetes 

 9,5 cm long (stage 20) showing the striated surfaces of their glandular 

 cells and the "Ersatzzellen" (ec). 



Fig. 36. a Longitudinal and b cross-section of a pronephric 

 funnel for the same Ammocoetes. 



Plate 4. 



Fig. 37. Stage 3. "Cuneate" transverse section, being the seven- 

 teenth section in front of the pronephros and in the branchial region, 

 showing the sclerotome (scT) as a hollow diverticulum and an aborted 

 diverticulum of the somatic mesoderm (som) corresponding in position 

 and appearance to the young pronephric tubules further back. 



Fig. 38. Stage 3. Cross-section through a pronephric diverti- 

 culum showing its origin in the thickened somatic layer. 



Fig. 39. Stage 3. Cross-section through the fourth pronephric 

 tubule (pron) showing the sclerotome (scl) arising as a hollow diverti- 



