386 Dr. Reid on the Development of the 



pear to be formed of a soft texture without any definite struc- 

 ture, having minute cells or nuclei scattered at short intervals 

 through it. Some of the embryos escape from their case-mem^ 

 hranes 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 hg. 15. The ciliated discs (c) and/oo^ {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 discs 

 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 difierent 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 lai-gest 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 oi 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 intestine ; 

 fig. 18 is a view of the left side, in 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 (i). 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 



