22 
The Embryolog'y of Patella. 
embryo. As they increase in deptk, their slit like openings 
come to lie parallel to and directly below tlie velum upon the 
ventral side of the embryo. The invaginations are directed for- 
wards and inwards and form two narrow-mouthed sacs which 
afterwards lie against the inner wall ot the velum. PI. IV, Fig. 45. 
On this account it is seldom that the same series of cross sections 
will show both the invagination and its opening equally well. In 
PI. IV, Fig. 53—55, which represent three consecutive sections of 
the embryo cut not quite parallel to the plane of the velum, the 
cavity of the auditory sac with its external opening can easily 
be studied. The inner ends of the invaginated sacs become clothed 
with mesoderm cells. With the closure of the external opening the 
sacs become entirely separated from the ectoderm and assume a 
globular form, while the cavity, which was never very large, is 
now alraost entirely obliterated by the increase in depth of the 
cells which constitute its walls. 
Meantime the auditory sacs have left their primitive position 
beneath the velum and become permanently located in the cavity 
of the foot. 
The velum has not materially changed those characte- 
ristics which we have already described in a previous section. 
It still consists of a central row of large, laterally compressed 
cells with a broad central band of cilia, flanked on either side 
by a row of smaller less uniformly regulär cells supplied with 
a narrow band of much smaller cilia. We have already called 
them the anterior and posterior support cells of the velum, on 
account of the manner in which they apply themselves to the 
walls of the larger cells forming a kind of support for them. The 
peculiar thickening of the posterior row of cells, PI. IV, Fig. 46, 
already described can no longer be observed. 
The embryo-cap has become greatly reduced in high during 
the latter stages, hardly projecting above the level of the velum; 
at the same time it becomes laterally compressed and assumes an 
oval form, when seen from above or in cross sections, in order to accom- 
modate itself to the opening of the shell, which has a similar shape. 
At the sides of the apical plate and somewhat dorsally 
placed appear two clear refractive spots formed by the projecting 
rounded ends of four or five cells of the epidermis; PI. V, 
Fig. 57 e. s. After about twenty-four hours an irregularly shaped 
pigment spot appears in the place formerly occupied by these cells, 
PI. V, Fig. 66, e. s. 
( 170 ) 
