186 
cut off, all of which are ectomeres. Then a fourth set of 
four (4a, 4b, 4c, 4d) is cut off from the lower cells. 
One of these four cells (4d) is very large; it is 
the mesoderm cell, or somatoblast, and it soon sinks below 
the surface into the cleavage or segmentation cavity. The 
other three cells of the fourth quartette (4a, 46, 4c), and 
the four lower cells (44, 4B, 4C, 47)) are the endoderm 
cells, and they occupy a position at the lower or posterior 
end of the larva. At this time the larva consists of (® 
about 50 ectomeres, products of division of the cells of the 
first, second and third quartettes; (2) the seven endoderm 
cells mentioned above, and (3) a mesoderm cell. At the 
end of twenty-four hours* after fertilisation, the larva, 
when seen from the lower pole, has the appearance shown 
in fig. 73. The ectoderm cells are by this time more 
numerous (about 70 to 80), and the endoderm cells, dis- 
tinguished by the numerous yolk granules in their proto- 
plasm, are in this case thirteen in number, each of the 
seven primary endoderm cells, except 4c, having divided 
into two. Lying in the segmentation cavity, and not 
shown in the figure, are two mesoblast cells produced by 
division of the primary mesoblast cell (4d). The larva, 
now a blastula, is converted into a gastrula by growth of 
the ectoderm cells over the yolk-laden endoderm cells. 
About twenty-eight hours after fertilisation the anterior 
ciliated band is just recognisable and the larvee were found 
to be rotating slowly within the vitelline membrane. The 
stomodzal invagination had also made its appearance 
(fig. 74). Twenty-four hours later both the ciliated bands 
of the larva were well marked, a broad one just anterior to 
the mouth, and a narrow one near the posterior end of the 
animal, and by this time the rotatory motion was much 
*“ It is very probable that in shallow water, in sunlight, the 
development would be more rapid than it was in the cool laboratory. 
