The Embryology of Patella. 
2L 
shell finally becomes nautiloid owing to its more rapid growth 
on the dorsal side. 
The anal cells still occupy the posterior end of the embryo; 
they are triangulär in sbape, the bases being directed inwards. 
The apices of the cells have developed a small plate-like pro- 
jection from which a bunch of long motionless hairs takes its 
origen ; PI. Y, Fig. 65. The nuclei of the cells are large and lie 
against the inner walls of the cells, as far as possible from the 
exterior. A flattened and elongated area of cells provided with a 
thick coat of fine and active cilia exists between the ones just 
described and the base of the foot; PI. Y, Fig. 57 and 58. As the 
edge of the shell gland encroaches upon the ventral surface, this 
ciliated patch of cells becomes bent forward with its outer sur¬ 
face against, or close to, the base of the foot, while the long stiff 
hairs of the anal cells are now pointed ventrally instead of 
posteriorly. The projection at the end of which are situated the anal 
cells now remains stationary, and as the posterior edge of the 
shell gland, which at this point is thick and composed of very 
large cells, continues to grow ventrally, it finally envelops the 
anal projection, leaving only the ends of the anal cells with their 
long hairs projecting beyond the edge of the mantle; PI. V, 
Fig. 58. The cavity of the shell gland has now entirely dis- 
appeared, the former thick floor of the cavity has become very 
thin and composed of flattened lenticular cells; PI. V, Fig. 65 
and 58. 
The foot has increased very much in prominence in the last 
stage ; its flattened anterior surface has approached so close to 
the velum that it almost entirely closes the opening to the Oeso¬ 
phagus and completely conceals the mouth from view. 
The posterior surface of the foot, which at first was convex, 
now becomes flattened and developes a thin horny operculum with 
which the mouth of the shell can be closed. Even at this late 
stage, if we observe the foot from above or below, a small inden- 
tation will be seen in the median plane marking the place where 
the blastopore passed between the two lateral swellings, the sub- 
sequent fusion of which gave rise to the foot. 
The auditory organs appear shortly after the closure of the 
blastopore as two shallow folds of the ectoderm, one on either 
side of the oesophagus, just below the velum; PL III, Fig. 42. 
These folds at first lie nearly parallel to the long axis of the 
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