364 RUTH B. ROWLAND 
coiling of the tubule, the early increase in size of the kidney 
being limited mainly to this region, between the connecting 
tubule and the proximal end of the segmental duct (figs. 5 and 
6, X to y). This eventually forms the ventrolateral region of 
the fully formed organ, which will again be referred to in con- 
nection with the discussion of edema. 
In still later stages^ the nephrostomal canals and their connect- 
ing tubule also elongate, and are thrown into loops and folds 
(fig. 6, l.t.) , retaining their dorsal position and extending shghtly 
laterally over the coils already formed. This portion may 
then be termed the dorsolateral region, as contrasted with the 
ventrolateral portion already mentioned. That part of the 
tubule which is a direct continuation of the segmental duct 
never becomes strongly convoluted, but retains its original posi- 
tion along the ventrolateral boundary, slanting obliquely toward 
the dorsal surface over the kidney from the anterior margin. 
Minor folds may occasionally occur along its course. Increase 
in growth is outwardly evidenced by a more and more pro- 
nounced swelling in the pronephric region. Operated speci- 
mens may be easily distinguished from normal animals, even 
after healing is complete, through the absence of this thickening 
on the operated side. Posterior to the pronephric coils, the 
segmental duct extends backward along the body just below 
the ventral surfaces of the muscle plates. The junction between 
pronephric coils and the proximal end of the segmental duct is 
always in the immediate region of the posterior funnel. With 
the subsequent downgrowth of the myotomes the formation 
of the shoulder-girdle and anterior limb buds, the pronephros 
becomes partly covered, and comes to lie deeper in the body, 
and nearer the midline of the embryo. The edges of the myo- 
tome also extend downward over the segmental duct, making its 
removal in this stage extremely difficult. 
^ Excised kidneys in the older stages may be slightly stained, and the capsular 
nuclei, thus made visible, removed. The tubules may then be easily uncoiled 
for observation in water or weak alcohol. Oil is an unsatisfactory medium, both 
for examination and preservation, as it not only increases the brittleness of the 
tubules, but renders them too transparent for clear definition. 
