BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
have taken place since the stage shown in Plate XI. Fig. 71, has come 
about chiefly by an invagination, independent of cell division ou the 
aboral surface, which has carried inward the endoderm and mesenchyme 
cells, has left at the margin of the wide-open blastopore tho anterior 
chorda cells and the muscle cells, and has brought plainly into view, 
outside the neuro-muscular band, another row of cells from the ecto- 
dermal surface extending round the entire margin of the embryo. A 
certain number of cells at the anterior end of this new ring is destined 
to serve the same purpose as the anterior segment of the neuromuscular 
band ; this fact is indicated in the figure by stippling. 
A clearer idea of the changes just sketched in outline may perhaps be 
had from an examination of cross sections. In Figures 73-77 are rop- 
resented five sections through the region of the blastopore of an embryo 
a little more advanced than the one shown in Figure 72. The approxi- 
mate position of the sections in the embryo is indicated by horizontal 
lines at the margin of Figure 72. In the endoderm cells the long 
deferred division leading to the seventh generation has at last been 
accomplished (Figs. 73-77). The endoderm cells accordingly number 
twenty, and their nuclei are greatly reduced in size on account of tho 
recont. division (of, d™*, Wig. 66, Plato X., with 455 dí, Wig, 75, 
Plate XL). The columnar form of the mother cells is retained by 
^ 
their descendants. 
It has been already stated that the spindles in the endoderm cells 
were ab the recent division approximately horizontal in position. It is 
evident, therefore, that before the accomplishment of division the attrac- 
tion spheres must have shifted from the position which they were seen 
to occupy in Plate X. Figs. 66-68, for otherwise the spindles would 
have stood vertically, and a two-layered arrangement of the cells would 
have resulted, such as we shall see does occur in the case of the mesen- 
chyme cells. No mechanical explanation of this change in the position 
of the attraction spheres in the endoderm cells offers itself, The longest 
axis of the cells appears to be continuously the vertical axis, yet the 
spindles form in a direction transverse to this in every instance. Van 
Beneden et Julin's 86) Figures Le and 2¢ also show spindles occupy- 
ing the short axis of the endoderm cells in the case of Clavelina. 
Considering now the mesenchyme cells we see (Fig. 74) that D Horas 
(Fig. 71) have divided in such a manner that à small superficial cell is 
separated in each case from a many times larger decp-lying sister cell 
(cf. Fig. 74, D878 915, Os, (7835 
in the inequality of its products is foreshadowed for the next anterior pair 
311412 
). A division similar in direction and 
of mesen’ iymo cells (Fig. 75, 07%, 742%, in which the spindles lie much 
