KIDNEY IN RELATION TO WOLFFIAN BODY IN THli CHICK. 



81 



muscle-plates persisted later than had been described. 

 Its connection with the ventral dilatation cf the muscle- 

 plate cavity is carried ventral wards as far as the outer dorsal 

 corner of the segmental duct ; so that it appears as a canal 

 opening into the body-cavity just internal to the segmental 

 duct, and thence curling round its dorsal wall to open into 

 the muscle-plate cavity. The ventral outer wall of this 

 passage is formed of large columnar cells, the inner and 

 dorsal wall of much flatter cells (woodcut, fig. 2), as seen in 

 transverse sections. 



Fjg. 2.1 — Transverse section through a younjr embryo of Scyllium. nip. 

 Muscle-plate. sL Segmental tube. ad. Segmental duct. c/i. 

 Xoiochord. sji. c. Alimentary canal. 



At the next stage of development the passage becomes 

 quite separated from the muscle-plate cavity, and now lies 

 as a blind tube, opening into the body-cavity internal to the 

 segmental duct, its blind outer end being applied to the 

 ventral dilatation of the muscle-plate body-cavity (woodcut, 

 fig. 2). This blind tube is the commencement of a segmental 

 tube. 



I have traced it through a series of older embryos to the 

 fully-formed segmental tube. The adjoining woodcut (fig. 2) 



^ Fig. 2 is from j\Ir. Balfour's forthcoming treatise on ' Comparative 

 Embryology,' It is copied from a figure in his ' Monograph on the 

 Development of Elasmobranch Fishes.' 



