52 
dorsal surface or nutritive pole of a somewhat older egg,, 
showing the two spherules of the endoderm uncovered by 
the ectoderm. The flattening of the embryo at the ends 
of the vertical axis, which had made its appearance at the 
stage shown in Figure 27, has now become more pronounced,, 
and the body is nearly disk-shaped, with its dorsal and ventral 
surfaces flattened and parallel. The two endoderm cells now 
divide up, and a short time after” the stage last figured they 
are six in number, as shown in Figure 30, which is a view 
of the ventral surface ; the dark endoderm cells being vis- 
ible through the more transparent ectoderm. Figure 31 
is an optical longitudinal section of a somewhat older em- 
bryo, represented with its dorsal surface to the right, and its 
anterior end above. The flattening of the upper and lower 
surfaces is well shown in this figure. At about this stage, or 
a little earlier, the ectoderm and endoderm separate from each 
other, and a well marked segmentation cavity, or, more prop- 
erly, a body cavity, is now clearly visible between them. The 
endoderm has now T divided up into a number of large spher- 
ules, forming a layer which is pushed in towards the ectoderm, , 
so that the dorsal surface is no longer flat, but saucer-shaped, 
thus forming a wide, shallow cavity, the primitive digestive* 
cavity, g. On the ventral surface the ectoderm cells now carry 
a few short scattered cilia, and the embryo begins to swim 
or rotate a little. 
It now undergoes considerable change of form, and in a 
few hours it presents, when seen in a side view, the form 
shown in Figure 32. 
This is a surface view of the left side, and Figure 33 is an 
optical longitudinal section of the same embryo. This stage 
is of great importance in the attempt to raise the young from 
artificially fertilized eggs, for the velum now makes its appear- 
ance, and the embryos swim to the surface of the water, 
where they form a dense layer, which can be siphoned off into 
a supply of pure sea water, leaving the dead eggs behind,. 
The outline at this stage is very irregular, but perfectly defi- 
nite and characteristic, although the great activity of the em- 
