The Embryology of Patella. 
7 
be seen, PI. I, Fig. 15 and 16, which during the first of tbe present 
period increase rapidly in size growing principally inwards toward 
tbe animal pole, wbile in doing so tbey encroach more and more 
lipon tbe Segmentation cavity and finally nearly obliterate it. 
Pb I, Fig. 15, shows a snrface view of tbese four cells, wbile 
PI. I, Fig. 16, is a section tbrongb an embryo of tbe same stage. 
An optical seetion of tbe embryo at tbe end of Segmentation sbows 
that tbese cells are wedge-sbaped, wbereas at tbe beginning of 
tbe gastrular stage tbey bave increased in size at tbeir inner 
ends, assuming an oval form, PI. I, Fig. 17 and Pb II, Fig. 20, 
wbicb immediately distinguisbes tbem from tbe surrounding cells 
of tbe ectoderm. 
As tbey continue to increase in lengtb tbeir inner extremities, 
wbicb are relieved from tbe lateral pressure of tbe adjacent cells, 
expand into peculiar club-sbaped cells, wbicb nearly fill tbe Seg¬ 
mentation cavity. PI. I, Fig. 18. 
It is dnring tbis increase in size and inward-growtb of tbe 
four primitive endoderm cells, tbat at first two and later four 
or five sligbtly enlarged cells at tbe apical pole become furnisbed 
witb tufts of sbort cilia, tbus forming tbe beginning of tbe apical 
plate; at tbe same time tbe velum is established by tbe appear- 
ance of similar tufts of cilia upon eacb one of a double row of 
cells extending around tbe equator of tbe embryo. Externally tbis 
double row of cells cannot be recognised as such, for tbe cells con- 
stituting it are only distinguisbed from tbe otbers by tbe presence 
of tbe cilia, PI. II, Fig. 21. A longitudinal section of the embryo at 
tbis period, PI. I, Fig. 17 and 18, sbows however tbat in reality 
the cells in t^uestion and tbeir nuclei are considerably larger than 
those of tbe remainiüg ectoderm cells. 
Tbe embryo, wbicb at tbis early stage is already provided 
with the rudiments of the velum and apical plate, can only roll 
about on the bottom of tbe glass or whatever vessel it may be 
in. It is not able to swim freely in any direction, until tbe 
growth of tbe post-velar portion has carried tbe centre of gravity 
out of, and beliind tbe centre of tbe equatorial plane in wbich 
tbe velum lies. 
It is important tbat we should have a correct idea of tbe 
general shape of tbe embryo and the relation of its main axes, 
before describing the further cbanges which take place. The 
embryo, as we bave seen, is spherical. Tbe four endoderm cells are 
situated at tbe vegetative pole, and opposite is tbe animal or 
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