264 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the caudal endoderm cells, i. e. are resolved into mesenchyme at a later 
stage. 
Figure 102 (Plate XIII.) represents a section through what probably 
was the region of the blastopore. In it is seen the most posterior pair 
of endoderm cells (cf. Plate XII. Fig. 94). Since the stage last exam- 
ined (Plate XII. Figs. 93-97) the chorda cells have closed together into 
a single plate in this region, and the chorda fundament has grown farther 
back in the embryo. The nerve cells which lay at each side of the 
blastopore (Plate XII, Fig. 94) have also met in the median plane to 
form a single plate, which is now closing into a canal. A real canal is 
never formed posterior to the blastopore, though the norve cells in that 
region potentially form one. 
Figure 103 (Plate XIII.) represents the second section anterior to the 
one shown in Figure 102; Figure 104, the second anterior to that; and 
Figure 105, the fourth anterior to that. It will be seen that the muscle 
cells which in the series last examined (Figs. 93-97) were aggregated 
chiefly behind the blastopore, have now extended themselves not only 
posterior, but also anterior, to the blastopore. They extend as far for- 
ward as the next section in front of the one represented by Figure 103, 
i e. through three sections anterior to the blastopore. They have 
pushed before them the mesenchyme, which in this series first appears 
in the section shown 1 
i Figure 103. The chorda fundament has mean- 
while moved toward the posterior end of the embryo. It now extends 
two sections behind the blastopore and overlies the small posterior me- 
senchyme cells (Wig. 101, cf. Plate XII. Fig. 93). Accompanying the 
changes just mentioned, has come a diminution of the diameter of the em- 
bryo at its posterior end, which is already elongating to form the tail 
region. 
The mesenchymo extends forward of the section shown in Figure 103 
through six sections. The medullary plate extends forward two or three 
sections farther still. The endoderm consists of a double row of cells 
extending forward underneath the chorda as far as the section seen in 
Figure 104, in which four endoderm cells are found; the arrangement 
there shown has been derived from that shown in Plate XII. Fig. 96, 
and still earlier in Plate XII. Fig. 91, by the meeting in the median 
plane underneath the chorda of the more laterally placed endoderm cells, 
Later, these four cells, or their descendants, will move apart so as to en- 
close between them the lumen of the posterior portion of the digestive 
tract. Anterior to the section shown in Figure 104 the endoderm 
rapidly increases in amount, while the chorda and mesenchyme diminish. 
