362 
RUTH B. ROWLAND 
The general methods employed in operating are so well known 
that no detailed description of them is necessary here. The 
special technique required in removal of the pronephros and in 
the construction, and measurement of the models will be described 
in later sections. 
The pronephric swelling is one of the earliest and most clearly 
defined of the developing organs. Its position may be accu- 
rately located at a stage not long after the closing over of the 
neural folds, when the first eight pairs of muscle plates may be 
seen, and, like these, it differentiates in an anteroposterior 
direction. It is found in the region immediately underlying the 
Figs. 3 and 4 Embryos showing the first looping of the tubule, due to rapid 
growth of the cells just posterior to the nephrostomes. Figure 3, a-a, original 
axis of the pronephric rudiment. Figure 4, 6-6, direction of first bend of the 
tubule. 
third and fourth myotomes as a bulbous thickening tapering 
posteriorly into a short thickened ridge. Operations at this 
period, although having the advantage of not interfering with 
developing nerves or blood-vessels, are inadvisable, since the 
mesoderm is still so compact that excision results ahnost invari- 
ably in the removal of more than the pronephric rudiment. In 
succeeding stages, the delimitation of the segmental duct pro- 
gresses, and the original bulbous enlargement becomes pitted in 
in two places on its coelomic border, establishing the nephros- 
tomal openings into the anterior and posterior funnels. These 
two openings lie opposite the midline of somites three and four. 
At the same time the tubule, which originally lies along the 
longitudinal axis of the body just below the muscle plates (fig. 
