406 REPORT—1863. 
spherical corpuscles, with still more minute granules. When the vitellus 
has thus become broken up into a great number of minute spheres, it is 
evident that the most superficial of these spheres have arranged themselves 
into a distinct stratum, consisting of a single layer of spheres, and completely 
enveloping the more internal parts (fig. 20 G). 
We next find that the spheres composing this layer have increased in 
number, while at the same time they have become longer in the direction of 
the radius of the ovum, and now form a rather thick layer of undoubled cells, 
arranged with their long axes perpendicular to the surface of the oyum, 
having their sides in close contact and investing, as with a continuous wall, 
the whole interior of the mass (fig. 20 H). 
It is impossible not to see in the entire process here described an exact 
parallelism with the early stages in the development of the mammalian 
ovum, while the superficial layer of cells, to the formation of which we have 
just arrived, must be at once recognized as the representative of the blasto- 
derm of the mammal*. 
The nuclei, which were previously visible in the cleavage-spheres, have 
now ceased to be distinguishable, while these spheres at the same time show 
a distinct investing membrane. In fact, on now carefully breaking down the 
ovum under the microscope, its interior is found to consist entirely of loosely 
aggregated cells, some spherical, some more elongated, and all with a more or 
less copious endogenous brood of secondary cells within them (fig. 20 H). 
The external enveloping layer having now attained a considerable thick- 
ness, and a well-defined differentiation between it and the more internal 
parts having been established, the ovum begins to elongate itself, and at the 
same time the interior has undergone a further change; for we no longer 
find in it the large mother cells with their endogenous brood, but a multi- 
tude of small, free, clear vesicles of various sizes mingled with the minute 
granules, which have all along formed a part of the constituents of the ovum. 
At this point we may conveniently, though somewhat arbitrarily, designate 
the developing body as the “embryo.”’ We find now that one end of the 
oval embryo begins to be prolonged beyond the rest, upon which it becomes 
bent back as it continues to elongate itself (fig. 20 1). By this time the 
embryo has become endowed with evident contractility, as manifested by slug- 
gish changes of contour. 
Shortly after this, the embryo escapes from its confinement by the rupture 
of the walls of the gonophore, when it speedily straightens itself and is 
discharged in the form of a long conical body through the summit of the 
gonangium into the surrounding water (fig. 20 K). 
We now find that its whole surface is clothed with vibratile cilia, by whose 
aid it moves slowly along the bottom of the vessel, while the cells and gra- 
nules which occupied its deepest parts seem to have undergone a kind of 
liquefaction, resulting in the formation of an elongated cavity in the axis of 
the embryo, which is thus, at that period, a nearly cylindrical sac, without, 
as yet, any appearance of a mouth, but with an endoderm and ectoderm 
already differentiated, while multitudes of very minute elongated-oval bodies, 
with a high refractive power, soon make their appearance in the ectoderm ; 
these are most probably thread-cells, though no sign of a filament can as yet 
be discovered in them. 
We have thus arrived at the ciliated and locomotive stage of the embryo. 
* The comparison of the structure of the Hydrozoa to the early stages in the development 
of the highest animals has been very distinctly made by Professor Huxley (Oceanic Hydro- 
zoa, p. 2). j 
