28 WILLIAM MORTON WHEELER, 



the intertubular ramifications of the cardinal vein, thus permitting a 

 flow of blood from the aorta through the glomus to the cardinal vein. 

 There can be little doubt that the pronephros is at this time actually 

 functioning. This is indicated by the increased accumulation of pig- 

 ment in the organ and in the adjacent peritoneum of the body wall. 

 The pronephros of the Petromyzon embryo is so near the branchial 

 region that its relations to the vascular system differ from those of 

 other Craniota. In these the pronephros lies behind the heart and 

 the ducts of Cuvier, and therefore in the region of the posterior 

 cardinal vein. In Petrompzon, on the contrary, the pronephros 

 develops in front of the ducts of Cuvier and is therefore associated 

 with the anterior cardinal vein. Goette justly emphasizes this pecul- 

 iarity, although it has been noticed by previous investigators. By 

 the time the yolk is exhausted in the cells of the mesenteron and 

 the little Ammococtes 7 to 7,25 mm long (stage 12) begins to feed 

 on diatoms, etc., the pronephros, especially its convoluted collecting 

 duct, extends some distance back into the lumen of the Cuvierian 

 duct of the same side. 



e) The Pronephros of the Ammocœtes. 



Of the development of the pronephros in the Ammocœtes I have 

 studied two sets of stages; the earliest comprising larva' 8 — 26 mm 

 long (stages 13 to 17). In these the mesonephrus is not yet well- 

 developed. In the second set I have sectioned the pronephros of an 

 Ammocœtes 9,5 cm long (stage 20) and that of one 17 cm long 

 (stage 21), ready to undergo metamorphosis. The mesonephros in 

 these two specimens was well-developed and functional, whereas the 

 pronephros, especially in the older Ammocœtes, showed unmistakable 

 signs of atrophy. 



Stages 13 to 17, which may be considered first, show little advance, 

 in the development of the pronephros beyond stage 12, except that 

 the organ has grown somewhat in size with the larva. In the older 

 specimens 22 — 26 mm long the pigment is more abundant and the 

 convolutions of the tubules are more distinctly embedded in the car- 

 dinal veins, so that only the nephrostomes protrude into the body- 

 cavity. The glomus is somewhat more constricted at its base, although 

 it still preserves an elongated pyriform or subtriangular outline in 

 cross-sections, and is attached near the bas(; of the pronephric lobe. 

 From this stage 1 shall pass directly to the Ammocu'tes 9,5 cm long, 

 a stage in which the pronephros reaches the hight of its functional 



