260 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Figure 98 by the five vertical lines 93-97. Figure 93 shows a section 
posterior to the blastopore. It passes through one (ms’ehy.) of the small 
posterior mesenchyme cells, 07%, 073 (cf. Fig. 88), the other lying in the 
next section posterior to this. The interior of the section is filled with a 
solid mass of muscle cells, or more properly nerve cells and muscle cells ; 
for it is highly probable that the four most dorsally situated of these 
cells, which form a group not quite covered in by the ectoderm, are to be- 
come part of the nerve cord of the tail (cf. Plate XIII. Figs. 99-101, n.). 
However, they are not distinguishable in histological characters from the 
more laterally and ventrally situated cells of the section. Cell division 
has recently occurred in the ectoderm, which plainly is soon to cover in 
completely the nerve cells in this region of the embryo. The muscle 
cells have evidently been reduced in size by division since the stage 
shown in Figures 88 and 89. 
The second section anterior to this is shown in Figure 94. It is the 
only section of the series which passes through the blastopore, now 
reduced almost to a slit. 
The blastopore is bordered on each side dorsally by a large nervo (N) 
cell, n. (cf. Fig. 90). Ventral to the nerve cells lie the posterior chorda 
cells, ed., lateral and still ventral to which are muscle cells. The most 
posterior pair of endoderm cells lies underneath the open blastopore, and 
a single small mesenchyme cell lies deep down in each half of the section. 
The second section anterior to the blastopore is shown in Figure 95 ; 
the second section anterior to that, in Figure 96 ; and one situated still 
two sections farther forward, in Figure 97. 
In Figure 96 the medullary plate is not at all depressed at its centre ; 
it consists of four large cells closely packed together and columnar in 
form. In Figure 97 the medullary plate is not even flattened, but con- 
forms to the evenly rounded contour of the embryo in that region. It 
consists of six cells sharply distinguished from the cells of the ectoderm 
in stainability, though the size of the more lateral ones is not materially 
different from that of the ectoderm cells, The chorda plate has dimin- 
ished to a breadth of only three cells in Figure 96, and is entirely want- 
ing in Figure 97, where endoderm cells occupy the space dorsal to the 
archenteron underneath the medullary plate. The mesenchyme bands 
cover considerable area in Figure 96, but are reduced to a single cell on 
each side of the body in Figure 97, from which it is seen that in this 
region the interior is nearly filled with a solid mass of endoderm. The 
section represented by Figure 97 lies well toward the anterior end of tho 
embryo, as is indicated by the rapidly diminishing size of the sections. 
