386 Dr. Reid on the Development of the 
pear to be formed of a soft texture without any definite struec- 
ture, having minute cells or nuclei scattered at short intervals 
through it. Some of the embryos escape from their case-mem- 
branes at an earlier stage than this, and their structures are 
consequently more opake. 
We now return to the earlier stages of the development. 
At the end of the ninth day the embryo had the appearance 
represented in fig. 15. The ciliated discs (c) and foot (d) were 
now distinctly seen, the central mass had separated itself to a 
greater distance from the shell (a), except at the upper part, 
and the portion within the shell had arranged itself into four 
imperfectly defined lobes, which are readily recognised to be 
earlier conditions of the structures described in the embryo 
when it is about to leave the case-membrane. The ciliated dises 
were however still small, but the embryos had now a greater or 
less degree of motion ; some performing a rocking motion, and 
others more advanced were rotating slowly round the interior of 
the case-membrane. Besides the large cilia on the margins of 
the ciliated discs, smaller cilia were observed on different parts of 
the upper end of the embryo. The mouth was distinctly seen, 
the hard plate on the lower surface of the foot had begun to form, 
and the transparent cells in the base of the foot were seen on 
subjecting the embryo to pressure. A layer of the minute cells 
or nuclei covered the inner surface of the shell, giving it a con- 
siderable degree of opacity. From the ninth to the eleventh day 
the ciliated discs had become more developed, more separated from 
each other, and much more moveable. The largest of the four 
lobes of the body had arranged itself into a stomach and intestine, 
and occasional contractile movements were seen in these; and 
the transparent cells in the base of the foot were now very obvious, 
On the twelfth day the embryo had assumed the appearances 
represented in figs. 16, 17, 18 and 19, and all the parts described 
in it at the time of its leaving the spawn were now distinctly seen. 
Fig. 16 is an anterior view, showing that the two large cells at 
the side of the stomach are at this period connected by a ridge 
running across the front of the gullet, and which afterwards nearly 
disappears. The larger or left cell does not at this period pro- 
ject so much in front of the stomach, and the right cell is larger 
and. lies near the anterior edge of the stomach. Fig. 17 is a view 
of the right side, showing the whole course of the imtestine ; 
fig. 18 is a view of the left side, m which the commencement of 
the intestine (i), curving itself to the right side, is the only part 
of that tube seen ; and fig. 19 is a view of the posterior surface, 
showing the tortuous course of the intestine (7), The cilia on the 
inner surface of the stomach and intestine are now also visible, 
The course of the gullet is not yet distinctly seen, and all the 
