8 
The Embryology of Patella. 
apical pole, at which point we have seen the enlarged ciliated eells 
which later form tlie apical plate. The main axis of the embryo 
therefore passes through the apical plate and the centre of the 
blastopore. 
The velum at first either coincides with, or when parallel to 
the equator, is situated nearer tlie blastopore than the apical pole . 
PI. II, Fig. 22. An indefinite number of radiating axes may be 
drawn in the eqnatorial plane having for their point of intersection 
the point at which the main axis of the embryo is perpendicular 
to the equatorial plane. The embryo is therefore a pure radial 
type. But this condition does not last long for two large cells 
soon appear, one of eacli side of the four endoderm cells, destroying 
the previous symmetry and transforming our embryo into a 
bilateral organism. These two cells may be seen in optical section 
in PI. II, Fig. 22 en. m 1 and en. m 2 ; and in actual section in PI. II, 
Fig. 23. Düring their present condition they may be called the 
e n d o-m e s o d e r m cells since the subsequent division of each of these 
cells-gives rise to two cells, one of which becomes the primitive 
mesoderm cell, and the other, after remaining in the mouth of 
the blastopore for some time, finally becomes pushed inwards by 
the narrowing of the blastopore, and forms one of the endoderm cells 
lining the cavity of the mesenteron. Bearing these conditions in 
mind we have only a few points to consider before we reach the 
next stage. 
On examination of PI. I, Fig. 17 and 18, and PI. II, Fig. 22, 
it will be seen that one or at most two cells only, intervene be- 
tween the velum and the large endoderm cells which fill the mouth 
of the gastrula. By the elongation and inward - gijowth of the 
endoderm cells the body of the embryo loses its spherical shape 
and becomes somewhat lengthened, to assist which, the ectoderm 
cells we have just pointed out increase in number and thickness. 
The cells which constitute the velum are large and wedge- 
shaped with convex outer surfaces, which later often become 
somewhat indentated at the point where the cilia originate. PI. II, 
Fig. 25. 
The embryo-cap, or that portion of the embryo anterior to 
the velum and including the future apical plate, consists of a small 
number of wedge-shaped cells decreasing slightly in size towards 
the summit, which is occupied by four or five cells provided with 
a few cilia, but otherwise indistinguishable from the surrounding 
cells. With the increase in length of the embryo, the embryo-cap 
( 156 ) 
