412 Miscellaneous. ; 
defined furrow between them, while the two lateral cells are sepa- 
rated from one another by the anterior and posterior ones. Thus 
the blastula already exhibits a bilaterally symmetrical structure. 
The hindermost macromere may be termed the endo-mesoderm 
cell, since it divides into two cells, of which the one retains the 
position of the posterior macromere, while the other, moving in the 
longitudinal axis, passes more towards the animal pole. This cell 
then similarly divides into two, but in the direction of the longitu- 
dinal axis, and the two cells thus produced are the primitive meso- 
derm cells, which lie next one another on both sides of the longitu- 
dinal axis, dorsally to the posterior macromere. 
After these processes have taken place the three other macromeres 
divide, but simultaneously with the fourth, which had superseded 
the endo-mesoderm cell, and furnish the endoderm cells of the wall 
of the archenteron. In the meantime the blastula flattens out 
dorso-ventrally, since the endoderm cells which have arisen from 
the macromeres, as well as the two primitive mesoderm cells, are 
surrounded by the ectoderm and pressed into the segmentation- 
cavity. With progressive flattening of the germ the archenteron is 
gradually formed by invagination of the endoderm, its walls being 
constituted by the progeny of the four macromeres. The flattening 
finally becomes so great that the segmentation-cayity is reduced to 
a cleft, while the blastopore forms an elongated oval which is situ- 
ated in the longitudinal axis. 
During this time the two primitive mesoderm cells, which on the 
invagination of the endoderm had come to lie at the hinder pole in 
the segmentation-cavity, have given rise to a mesodermic band on 
either side of the archenteron. Simultaneously with this the 
embryo has also lost its rounded form, and when seen from the 
ventral or dorsal surface appears as a spherical triangle with nearly 
equal transverse and longitudinal axes, and with the apex directed 
forwards and rounded angles. 
At the next stage the blastopore forms a long slit, which occupies 
the whole length of the ventral side. The communication between 
the archenteron, which possesses a tolerably wide cavity, and the 
exterior persists at about the middle of the blastopore, while the 
edges of the blastopore elsewhere grow together. The mouth 
proceeds directly from the persisting communication between the 
blastopore and the exterior. The first traces of the velum now also 
appear in the shape of a double row of clear ciliated ectoderm cells, 
which form a girdle directed obliquely to the longitudinal axis, and 
which bisects the longitudinal axis in the dorsal median line and in 
the ventral median line passes in front of the anterior end of the 
blastopore. The mesoderm has become bilamellar and forms a 
saccule on each side on the right and left, which pass into one 
another at the hinder pole and gradually grow out forwards and 
dorsally. The coelom lying between the two layers of the meso- 
derm is distinctly visible. 
The archenteron soon changes its shape. It is broader in front, 
with a wider lumen, and narrows towards the hinder end, whereby 
its lumen becomes correspondingly smaller. In lateral view it is 
