40 WILLIAM MORTON WHEELER, 



to the prooephric lobe, viz. on the mesial surface usually near its 

 ventral edge. In certain other respects, however, the glomeruli are 

 variable. Tn the extreme anterior portion of the mesouephros they 

 ai'e rounded or elliptical bodies, each with two or more funnels as 

 shown in Fig. 65. Occasionally two glomeruli fuse as in that figure. 

 In an Ammocœtes 9,5 cm long, with a much more extensive meso- 

 nephros, I find in the anterior portion of the organ four round glomeruli, 

 clearly separated from each other by spaces twice as long as the 

 glomeruli themselves. Then follows a long continuous cord-shaped 

 glomerulus extending through the greater portion of the organ. 

 Finally behind this follows another isolated glomerulus and an elong- 

 ated glomerulus again, about four times the length of the preceding. 

 In Fig. 59 is represented a sagittal section through a small portion 

 of the long cord-like glomerulus {mes. gl). It extends across the whole 

 figure and is supplied at intervals by arterioles from the aorta like 

 the one represented at gl. a. There can be little doubt that this is 

 a compound glomerulus arising from a fusion of several glomeruli 

 like those which remain isolated in the anterior part of the organ of 

 the younger Ammocœtes (Fig. 65). On all sides the ciliated funnels 

 of the mesonephros open into the cavity surrounding the vascular 

 cord. This cavity, however, represents a complex Bowman's capsule, 

 for it is divided into compartments as seen in Fig. 59, so that each 

 funnel has its own cavity. The discontinuity of the glomeruli at the 

 posterior end is shown in Fig. 61. Since in the older Ammocœtes 

 and the adult Petromyzon there is only one long continuous glomerulus, 

 I conclude that the posterior isolated glomeruli subsequently fuse with 

 one another and with the long glomerulus in front of them. The 

 disconnected glomeruli in the most anterior portion of the mesonephros 

 must disappear with the atrophy of that region during the growth of 

 the Ammocœtes. 



3) The variations in the diameter of the nephric tubules is well 

 seen in Ammocœtes 9,5 cm long by comparing the cross-sections of 

 the pronephric tubules and the tubules from the anterior and posterior 

 portions of the mesonephros. It is seen that there is a gradual 

 diminution in calibre from before backwards. In the anterior portion 

 of the mesonephros the tubules are only ^/^ or at most ^j^ the 

 diameter of the pronephric tubules, whereas in the posterior end of 

 the organ they are scarcely more than ^g or V4 ^^^^ diameter of a 

 pronephric tubule. Corresponding proportions are observable in the 

 funnels of the pronephros and in the anterior and posterior regions of 



