16 
The Embryology of Patella. 
distinguish it except by means of sections, on examination of 
which those cells will be readily distinguished as a single row 
of wedge-shaped ciliated cells closely applied to and supporting 
the very large qnadrate or wedge-shaped velar cells; PI. IV, 
Fig. 51 and 52, etc. 
The peculiar form and distinctness of this widened dorsal 
portion of the posterior row of Support cells appears soon after 
the blastopore has begnn to move toward the ventral surface and 
i s retained until after the evagination of the shell gland. 
The shell gland makes its first appearance shortly before 
the closure of the blastopore, as a plate of thickened cells; PI. III 
Fig. 41 sh. g., situated on and including almost the entire dorsal 
surface of the embryo posterior to the velum. A flattening in the 
onter ends of the cells and a large shallow depression in the 
centre of the plate are the first indications of the invagination 
which is formed there at the beginning of the next stage. 
The cells composing the embryo cap have during this stage 
increased in number and deoreased in size, assuming a more 
cuboidal shape, only those at tbe summit bearing cilia. The cells 
of the apical plate have also increased in number and become 
much elongated forming a lenticular thickening, the convex side of 
which is turned inwards. PI. IV, Fig. 49 and 50. As these cells 
increase in length tlieir nuclei, increasing in size and containing 
less stainable substance than before, leave their central position 
and move towards the inner ends of the cells. 
As the apical plate increases in size the onter ends of the 
cells composing it, which were at first rounded, become flattened 
and tinely granulär, while the nuclei, now more deeply located, are 
no longer visible from the exterior. Meantime two cells, one on 
each side of the apical plate, have become eonspicuous by retaining 
their rounded onter ends and by an increase in length which 
causes them to project some distance above the level of the sur- 
rounding cells. PI. III, Fig. 35, 36 etc. a. c. The projecting ends 
become iilled with a number of highly refractive granules and 
covered with extremely fine radiating hairs, which when seen 
upon the living animal are perfectly straight and motionless. 
The remaining cells of the apical plate, and also nearly all 
the cells of the anterior half of the embryo-cap, have become 
supplied with short and active cilia, which upon the two or three 
cells in the centre of the apical plate and exactly at the apical 
or animal pole, become enormously developed and form a tuft of 
(164) 
