The Embryology of Patella. 
11 
time they assume at the apex of the groove a decidedly circular 
arrangement, PI. III, Fig. 37 and 38. Meantime the diverging arms 
of the Y have become parallel enclosing a narrow and shallow 
furrow which deepens at its widened basal extremity, PI. III. Fig. 38, 
forming a round opening similar to the. one at the opposite end of 
the furrow , but smaller and not so distinctly marked. The furrow 
connecting the two pits becomes less distinct and soon after 
the posterior pit nF disappears leaving a long shallow groove 
to mark its former position, PI. III, Fig. 37 and 42; the 
anterior pit however remains and , becoming completely sur- 
rounded with prominent wedge-shaped cells, forms the opening 
of the oesophagus or mouth. At the same time the blastopore 
closes. 
The ectoderm cells, which formed the side walls and floor 
of the V shaped furrow, now constitute, the walls of the stomo- 
daeum or oesophagus. 
Just before the blastopore moved from its central position 
at the basal end of the embryo it was filled with four lai’ge cells, 
which had grown into and almost filled the Segmentation cavity, 
at the same time they underwent a division more or less perfect 
in a plane perpendicular to the long axis of the cells. The pro- 
ducts of this division remain for some time intimately associated 
with the parent cells, PI. II, Fig. 22, showing by their configu- 
ration and position the source of their origin. As the blasto¬ 
pore however moves towards the ventral surface, the cells in 
question become separated from the parent cell and assume rounded 
or somewhat distorted and compressed shapes in order to accom- 
. modate themselves to the limits of the Segmentation cavity, which 
they soon completely fill. At the same time the parent cells, which 
continue to occupy the blastopore, divide parallel to their long axes 
increasing the number of cells in the mouth of the gastrula to 
seven or eight. As the blastopore migrates t.oward the position of 
the future moutb, the productß of the first division of the four 
endoderm cells become less distorted and, as the embryo increases 
in length, arrange themselves symmetrically beneatli the apical 
plate. The products of each subsequent cross division of the cells 
which fill the, blastopore become smaller with each succeeding 
division, owing to the diminishing size of the parent cells. By the 
time the blastopore bas closed, the endoderm cells, which before 
were irregularly arranged and completely filled the Segmentation 
cavity, have now assumed some order in their distribution. A long 
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