The Embryology of Patella. 
9 
cells lose their wedge like form and become somewbat flattened. 
PI. II, Fig. 25 and 32. 
V. Migration of the Blastopore and Appearance of the Dorso-ventral 
Axis. 
At the end of the preceding stage the four primitive endo¬ 
derm cells had divided at their inner ends giving rise to a small 
nnmber of large cells, while the outer ends of the four original 
cells still filled the mouth of the gastrula. 
If, at the beginning of the present stage, we examine the 
blastopore it will be seen that the four large cells with which it 
was filled have now increased to seven or eight. In surface views 
of this part of the embryo, these cells are easily distinguished 
from the snrrounding ectoderm by their finely granulär appearance 
and the absence of nuclei, whereas the ectoderm cells are filled 
with fat globales and contain well marked nuclei. PI. II, Fig. 28 
and PI. III, Fig. 43. This difierence in the appearance of the 
two kinds of cells is easily explained by the examination of 
sections through the blastopore at this stage. Such a section is 
seen in PI. II, Fig. 25. In this example the number of free endo¬ 
derm cells is very small, the majority consisting of elongated cells 
distended at their inner extremities and closely packed in the 
gastrula mouth, which they completely fill. The nuclei are situated 
at the inner ends of the cells a long way removed from the 
outer surface, thus acconnt.ing for their absence in surface views. 
The outer ends of these cells are usually deeply stained and 
consist of very fine and uniformly granulär protoplasm extending 
inwards as far as the large vesicular nuclei. Beyond the nuclei 
the cells consist of protoplasm, which is reduced almost to a 
network by a great number of cavities, which in the living cells 
were probably filled with fatty material, now removed by the 
action of the reagents. 
In the same Fig. 25, en. 1 is a very large cell almost divided into 
two portions, the narrowest part connecting the two ends is filled 
with protoplasm, the granules of which have arranged themselves 
with their long axes parallel to the long axis of the cell. A 
second cell en 2 may be seen in the same section, that has com¬ 
pletely separated itself from the parent cell, which still lielps to 
fill the blastopore. 
Finely granulär protoplasm may be seen in cell en 2 marking 
the place where it was formerly connected with its parent cell. 
(157) 
