505 



Anterior end of the pronephric duct. 

 The front end of the pronephric duct is at first either an open 

 groove or a solid outgrowth from the nephrotome. When the process 

 of segmentation affects this region the outgrowth becomes nipped off 

 from the nephrotome at intervals, the remaining connections giving 

 rise later to the pronephric tubules. 



p.t. 



gl. c 



gl. e.r. 



Fig. VII. Diagrams illustrating the relations between pronephric and meso- 

 nephric tubules. Pronephric duct black ; nephrotome unshaded ; somite with trans- 

 verse shading ; lateral plate with vertical shading. 



A k B represent an anterior and posterior pronephric tubule at an early stage. 

 C & D represent the same tubules respectively as a later stage. E & F show 

 transition stages from a pronephric to a mesonephric tubule. 



gl. glomus; gl. ex. external glomerulus; gl. in. internal glomerulus; m.t. 

 mesonephric tubule ; p. t. pronephric tubule. 



