14 
The Embryology of Patella. 
Fis 1. 
pore does not lie in the cells filling the blastopore itself, but 
ent.irely outside of tbem and indeed in ihe cells wbich lie npon 
the dorsal side of the embryo. An un- 
expected help in locating and following 
the growtli of these cells was given in 
the appearance of two large cilia bearing 
cells, which appear verjr early on the 
dorsal side of the blastopore just about 
the time it begins its ventral movement; 
PI. II, Fig. 27 and 32, and PL III, Fig. 36. 
These two cells, whose outer ends are 
sharply granulär and covered with very 
fine motionless hairs, are situated close 
together one of each side of the median 
dorso-veutral plane and at first on the 
dorsal side. Only one row of cells sepa¬ 
rates them from the blastopore while two 
or three intervene between them and the 
velum. A flattening of these second inter- 
vening cells causes an increase in their 
superficial area, at the same time they 
increase in number by division. The 
large velar cells remaining fixed, this ex- 
tension of the dorsal surface, not being 
counteracted by a corresponding one on 
the opposite side, forces the blastopore 
and the anal cells from their original 
Position towards the ventral surface. This 
continues until the blastopore has reached 
the position indicated in PL II, Fig. 31 
and 32, when the movement is accelerated 
by growth in another region. By compa- 
ring PL II, Fig. 25, with Pl. II, Fig. 27 
and 32, it will be seen that the number 
of cells upon the ventral side has re- 
mainednearly the same while on the dorsal 
side they are not so deep, and have plainly 
increased in number and extent. 
After the anal cells have been car- 
ried by the growth of the dorsal surface almost to the basal 
end of the embryo, the cells between the blastopore and the anal 
( 162 ) 
