The development of tlie iiriiiogeiiital organs of the lamprey. 39 



cut in a siugle section, all of them radiating from the glomerulus in 

 the same transverse plane. 



c) Different Regions in the Mesonephros. 

 While the formation of the mesonephros throughout the whole 

 nephric lobe and from the time the animal is 12 mm long till it 

 undergoes metamorphosis at a length of about 17 cm, is essentially 

 the same, there are, nevertheless, several differences in the details of 

 the process in different parts of the nephric lobe. These differences 

 not only serve to reconcile the varying accounts of the authors who 

 have worked on Petromy^on, but they are also interesting from a 

 comparative and theoretical point of view. They may be distinguished 

 as 1) differences in the relations of the pronephric duct to the meso- 

 nephros, 2) differences in the development of the glomeruli in different 

 regions and 3) differences in the tubules. These points may be con- 

 sidered seriatim. 



1) The relations of the pronephric duct to the mesonephric tubules 

 are not very definite in the most anterior portion of the organ. This 

 is seen in Fig. 65 which represents the entire pro- and mesonephros 

 of a larva 22 mm long as seen from the mesial surface. The collecting 

 duct (cd) after leaving the pronephros above the heart, turns back upon 

 itself a few times and then runs posteriorly as the pronephric duct 

 (d) over the region occupied by the liver. Then it gradually descends 

 along the edge of the mesonephric lobe till it reaches the region 

 occupied by the contorted mesonephric tubules. Here it begins to 

 undulate, coming to lie now dorsal, now ventral to the mass of tubules 

 and their glomeruli (mes. gl) till it reaches the posterior end of the 

 developed portion of the organ, when it runs straight to the cloaca. 

 When the more posterior portion of the mesonephros develops, the 

 duct no longer undulates but is always found in the ridge of the 

 nephric fold below the tubules as seen in cross-section in Fig. 67. 

 This position it also preserves in the adult. Fig. 67 should be com- 

 pared with Fig. 63 through the anterior portion of the mesonephros 

 of a younger specimen where the glomeruli and the funnels on both 

 sides are ventral to the duct. 



2) The glomeruli, arising throughout from the mesonephric band 

 are more constant in their position than the duct. The dislocation 

 of the band with respect to the duct as seen in Fig. 66 is slight and 

 probably not constant in all larvœ. The glomeruli occupy the same 

 position with respect to the mesonephric lobe that the glomus does 



