592 SYDNEY J. HICKSON. 
possible to study all the phases in the formation of the ectoderm 
by the examination of one good section. 
The ectoderm is formed in this manner: as soon as the 
nuclei have taken up their position in the peripheral proto- 
plasmic membrane, the latter increases considerably in thick- 
ness. The protoplasm then breaks up into blocks, each block 
containing a single nucleus (fig. 21a). These blocks or cells, 
as they may now be called, are separated from one another by 
lacunar spaces. I can find no protoplasmic threads connecting 
them together nor any residue, so that I think we are justified 
in assuming that in life they are filled with a thin watery fluid. 
The ectoderm still continues to increase in thickness, and the 
cells are in consequence drawn out into spindles, isosceles 
triangles, and many other elongated shapes (fig. 218). At the 
external and internal extremities, however, the cells are 
continuous with one another. 
When the ectoderm has completely developed all round, the 
embryo is ready to escape. A passage is prepared for it by 
the absorption of the superjacent calcareous walls of the 
ampulla, and it escapes. 
From the commencement of the formation of the ectoderm 
to the time of the escape of the embryo no noteworthy change 
occurs in the structure of the central mass. It consists 
from first to last of a mass of spongy protoplasm bearing in 
its meshes numerous embryonic nuclei and yolk bodies. There 
is certainly a considerable increase in the number of the 
nuclei, and in the number of the vacuoles; but I cannot 
detect even in the oldest embryos any arrangement of these 
nuclei and the surrounding protoplasm to form an endoderm, 
no invagination of the primitive ectoderm, and nothing 
comparable to a process of delamination. 
V. The History of the Yolk-Spheres. 
It is clear from the description above and the figures given 
that the yolk of the ova of Allopora is not formed without, 
nor by buds from the germinal vesicle. Neither the cells of 
the chorion nor the endoderm cells of the trophodise exhibit 
