EARTHWORM. 221 
the two embryos are often twins formed from one ovum, separation 
taking place at the gastrula stage. 
The whole process of growth, until leaving the egg, lasts from two 
to three weeks, the time varying, however, with the temperature. 
The ovum is surrounded by a vitelline membrane, and is laden with 
yolk granules. Segmentation is slightly unequal (Fig. 114 (1)), and 
exhibits considerable variation even within the limits of a species. 
In about twenty-four hours a nearly spherical, one-layered blasto- 
sphere or blastula is formed. It consists of only about thirteen cells. 
During the next twenty-four hours the cells increase in number rapidly, 
but the blastula remains one-layered. Two cells lying together do not 
take part in this division ; they are rather larger than the rest, and their 
inner ends project into the cavity, and are soon cut off as daughter-cells. 
Gradually the large cells still undergoing division begin to sink in, and 
at last are quite included in the cavity (Fig. 114 (2)). Thus there arise 
two parallel rows of cells within the blastula, and these define the 
longitudinal axis of the embryo. This is the beginning of the mesoblast 
which forms all the muscles of the trunk, and which thus takes origin 
from two primary mesoblasts. 
After five to six pairs of secondary mesoblasts have been formed, the 
blastula begins to flatten, and to elongate, becoming an oval disc. The 
cells of the lower surface become clearer, and the hypoblast is thus 
defined. The cells of the upper surface are smaller, and become very 
much flattened ; they compose the epiblast. The mesoblasts lie side 
by side near one end, forming two rows extending forwards and down- 
wards, but divergent, because of the flattening of the blastula. The 
hypoblast now becomes concave, and thus the blastopore arises, 
occupying the whole of the lower surface (Fig. 114 (3)). The sides close 
in and the blastopore becomes a slit, which further closes from behind 
forwards, leaving only a small opening—the future mouth. During 
these processes the cells at the anterior tip of the blastopore, which 
will give rise to the pre-oral lobe, undergo no change, but the mesoblast 
has been active. 
As gastrulation proceeds, the mesoblast rows grow forwards and 
upwards, until they come near each other above the anterior tip of the 
blastopore, while their middle portions are carried downwards until they 
lie on the ventral surface. Over them the epiblast is thickened in two 
bands. Two longitudinal rows of epiblast cells near the anterior end, 
and ending behind in large cells, sink in just as the primary mesoblasts 
did. The thickening now extends ventrally until the two bands meet 
and, passing into the blastopore, form the stomodeum. Even before 
this the embryo has begun to swallow the albumen in which it floats. 
There are now two lateral bands of cells called the germ bands, 
composed of three layers (Fig. 114 (4)); outside is the thickened epi- 
blast, next the rows of cells which sank in, and innermost the meso- 
blast rows. The mesoblast rows have met in the middle line by 
dividing and widening out into a pair of flattened plates, but they still 
end behind in the two primary mesoblasts. Ccelomic cavities develop 
in the plates, and the anterior ends meet above the mouth. The 
epiblastic rows which sank in (there were eight of them, four on each 
‘side of the median line, and each ending in a large mother-cell) go on 
. 
