KIDNEY IN RELATION TO WOLFFIAN BODY IN THE CHICK. 165 
muscle-plates persisted later than had been described. 
Its connection with the ventral dilatation of the muscle- 
plate cavity is carried ventralwards 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. 
Fig. 2.J — Transverse section through a young embryo of Scytlium. mjj. 
Muscle-plate. s^. Segmental tube. sd. Segmental duct. c/i. 
Notochord, sp. v. 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 Mr. Balfour’s forthcoming treatise on ‘ Comparative 
Embryology.’ It is copied from a figure in his ‘ Monograph on the 
Development of Elasmobranch Fishes.’ 
