The Embryology of Patella. 
15 
cells rapidly multiply, thus increasing the distance between those 
two structures wliile the anal cells remain almost motionless at 
the basal end of the embryo. 
The three adjoining wood cuts will make these changes in 
the posit.ion of the blastopore easily nnderstood. 
Düring the present stage the velum has assnmed enormons 
proportions, and is composed of two rows of large cells encircling 
the embryo, and situated somewhat nearer the apical than the basal 
pole. The cilia, which upon their first appearance formed a tnft 
in the centre of each velar cell, increase in size and arrange them- 
selves in two continous bands passing through the centre of 
each row of cells. The nuclei are situated in the centre of the 
cells opposite the cilia, from which they are separated by a band 
of finely granulär protoplasm the width of which equals that of 
the band of cilia; PI. II, Fig. 27 and 32. As the embryo grows 
older, the anterior row of cells decreases in size and becomes 
hardly distinguishable upon preparation of the whole embryo. 
They form what we shall call the anterior Support cells of 
the velum. At first they are triangulär with their bases resting 
upon and conforming to the adjacent wall of the large velar 
cells; later they become much flattened and elongated, but still 
retain a small band of cilia on their outer surfaces. x ) 
The posterior row of velar cells increases in size and forms 
a single row of very large laterally compressed cells, whose cilia 
have also increased in size and form a rather broad and conti- 
nuous band passing through the centre of the cells. This band is 
very distinct in preparations of the whole embryo owing to the 
absence of fat globules in the velar cells at the base of the cilia. 
A third row of cilia bearing cells is formed on the posterior 
side of the large cells, and afterwards constitutes the posterior row 
of support cells. On the dorsal side of the embryo this latter row 
of cells becomes widened and especially well marked in prepa¬ 
rations of the whole embryo. The band is widest on the dorsal 
side, in the centre of which it becomes slightly indentated. PI. III, 
Fig. 36. It gradually decreases in width and distinctness as it 
passes towards the ventral side, where we are no longer able to 
3 I have often observed in young specimens that, wliile the velum consisted 
of only one row of cells on the dorsal surface, that on each side of the embryo the 
velum often developed two or even three rows of cells. I have not observed this 
condition in stages later than the closure of the blastopore. 
(103) 
