10 
The Embryology of Patella. 
Such endodermic cells, which have only recently been separated 
from those Alling the mouth of the gastrula, can always be distin- 
guisbed by the one sided position of this deeply stained patch 
of protoplasm, which afterwards becomes uniformly distributed 
throughout the cell. 
About the twentieth hour the blastopore no longer retains 
its central position at the basal end of the embryo. It gradually 
moves towards the future ventral surface retaining meantime its 
size and cbaracteristic appearance. In PL II, Fig. 27 is a section 
of an embryo of twenty-five hours where it will be seen that 
the centre of the blastopore is not equally distant from the velum 
on each side, a greater number of ectoderm cells intervening on 
the dorsal, than on the ventral side. PI. III, Fig. 40 is also a longi¬ 
tudinal section through the blastopore of an embryo 20 hours old, 
it will be seen that the gastrula mouth has become more ventrally 
placed and smaller, wliile it is filled with only two large cells 
and the small ends of two others. In PI. II, Fig. 31, it has become 
still more ventrally placed and slightly elongated. 
After the blastopore has reached about the position indicated 
in PI. III, Fig. 34, it decreases in size and occupies the bottom of 
a Y shaped hollow, which increases in depth as it decreases in 
width. The apex, which is the deepest part of the furrow, is 
directed towards the velum. As the groove narrows it grows 
deeper and finally it is impossible to distinguish in surface views 
the ends of the endoderm cells, which fi.il the mouth of the 
gastrula and pave the anterior floor of the deepening groove. 
Sections show its plainly that the blastopore, which is now very 
much reduced in size, occupies only a small part, the apex, and 
very deepest portion of this Y shaped furrow, PL III, Fig. 41. 
while the remaining part is occupied by ectoderm cells, which 
ascend abruptly to the surface in the wall next the velum, while 
at the opposite side the slope is more gradual. The enormous 
cells, which formerly filled the blastopore, have become extremely 
attenuated and, at their outer ends, reduced almost to lines in 
order to accomodate themselves to the now reduced limits of the 
blastopore. 
Before the pressure upon the surrounding cells has forced 
the ends of the endoderm cells out of the gastrula mouth and 
therefore permanently closed the blastopore, the ectoderm cells 
surrounding the V shaped pit have increased slightly in size and 
become conspicuous by their regulär order, while at the same 
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