38 WILLIAM MORTON WHEELER, 



connections between the thickenings, or nodes consist of nuclei regularly 

 arranged with their long axes at right angles to the long axis of the 

 band. The mesonephric band may be traced in larvae 

 22 mm long and older from the posterior end of the 

 mesonephros, however slightly this may be developed, 

 to witliin a short distance of the posterior termination 

 of the prone phric duct, and it is from this band that all 

 the mesonephric tubules arise in succession. 



The transition from the developed to the undeveloped portion of 

 the mesonephros is often quite abrupt, a fact which would indicate 

 that the addition of new tubules to the posterior end of the organ is 

 periodic. This is seen in Fig. 61, taken from a sagittal section through 

 the posterior end of the developed portion of the mesonephros of an 

 Ammocœtes 9,5 cm long. The pronephric duct (d) is cut lengthwise 

 in the lower portion of the figure, and above it the fatty tissue on 

 the right has been invaded by the convoluted mesonephric tubules 

 {mes). These are well-developed and open by means of deeply staining, 

 ciliated funnels (mes.f) opposite the glomeruli (mes. gl). In the right 

 hand portion of the figure the fatty tissue has retained its primitive 

 trabecular character and below are seen the rudiments of several 

 mesonephric tubules growing into the walls of the pronephric duct. 

 In one case two, in another three tubules unite to form a common 

 cord of cells which force their way between the paler cells of the duct. 

 In front of the single deeply staining tubule of this figure is seen a 

 more robust tubule (mes) which alone forms the transition from the 

 completed tubules on the left to the very young rudiments on the 

 right of the figure. Fig. 60 taken from a sagittal section through 

 the same kidney a little further back than Fig. 61, shows how from 

 three to five tubule rudiments, all starting in the mesonephric band, 

 may unite to form a common tubule perforating the wall of the pro- 

 nephric duct. The actual openings of the fully formed tubules into 

 the duct in tufts by means of common openings is not shown in any 

 of the figures. 



It was stated above that there is no definite relation between the 

 mesonephric tubules and the metameres of the body. In the youngest 

 Ammocœtes (12 mm), in which 1 find a small portion of the meso- 

 nephros established, I can count 12 funnels in the space of three 

 segments in the right mesonephros, and in the left 11 funnels in the 

 space of two segments. In older specimens the number of funnels 

 per segment must be even greater, since one often finds three funnels 



