MUSEUM OF COMPARATIVE ZOOLOGY. EL 
In Figures 26 and 27 (Plate IV.) is represented the region of the nephro- 
stome just prior to the beginning of its formation, and Figures 28 and 29 
represent the anterior region of the segmental duct in the same embryo. 
At this stage no trace of either structure can be seen, but a little later 
both appear. In an embryo about six hours older than the stage to 
which reference has just been made, these structures are in process of 
formation. In the corresponding protovertebre (Plate V. Figs. 31, 32, 
33) the nephrostome (pr’nph.) is in process of formation as an evagina- 
tion of the proximal region of the lateral mesodermic layers opposite the 
third protovertebra. The lumen of the nephrostome, continuous with 
the body cavity (ce/.), is plainly seen and persists very conspicuously 
in stages more advanced (Plate VI. Fig. 42, and Plate VII. Fig. 46). 
In the fourth protovertebra of the embryo in which the nephrostome 
is forming, a mass of cells is seen in a position corresponding to the 
nephrostome, but it is without any distinct connection with the proximal 
margins of the lateral layers. This mass of cells is also more or less 
detached from the principal part of the intermediate cell-mass, which 
now occupies a position under the chorda. In the next (fifth) proto- 
vertebra (Fig. 34) this structure is plainly recognized as the segmental 
duct (dt. seg.) ; it is independent of any connection with the lateral 
layers, and more nearly detached from the intermediate cell-mass than 
in the fourth protovertebra ; its cells are arranged in a peripheral order, 
and a faint lumen can be observed. The condition of the segmental 
duct for some distance back of the fifth protovertebra is the same as 
here, unless perhaps the development may be slightly more advanced 
than in the extreme anterior end. It appears, therefore, that the an- 
terior portion of the segmental duct is developed from the distal (lateral) 
margin of the intermediate cell-mass; that, as the uuclei arrange 
themselves to form the walls of the tube (Fig. 28), a lumen is formed 
and the whole is gradually cut off from the inner (axial) portion of the 
intermediate cell-mass, which passes to the axis of the embryo, becomes 
more voluminous, and, as we shall see, has a different fate. In more 
advanced stages the duct becomes more perfectly cylindrical, and its 
central area more nearly circular in cross sections. (Compare dt. seg. 
in Figs. 35, 43, 44, 49, 50.) 
The segmental duct retains its original position, but the proximal 
margin of the lateral layers passes toward the median line, and, as the 
entoderm closes in to form the tubular intestine, these layers press upon 
it from the two sides, and later clasp it, as may be seen by reference to 
Figures 34, 35, and 43. Hence it follows from these changes that the 
