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J. D. F. GILCHRIST. 
a stage showing six segments in section (PL 13, fig. 10) ; here- 
it was very small, and was occupied by a homogeneous sub- 
stance stained faintly with haematoxylin. A well-marked 
blastula is soon developed after this stage, for a section 
showing nine cells has a relatively large blastocoele (PL 13, 
fig. 11). Here the cells at one side appear larger than at the 
other. Other sections, however, show that the grouping of 
large cells at one point does not appear to be constant. 
In a blastula of fourteen cells in section (Pl. 13, fig. 12) 
one cell was observed entirely within the hitherto complete 
circle of cells. It seems from subsequent events that this 
arises by proliferation of an outer cell rather than by ingrowth 
of a cell. It marks, as will be seen in later stages, the 
posterior end of the developing embryo. 
The beginning of a further change is seen in a blastula 
of about twenty-nine cells in section (Pl. 13, fig. 13), in which 
a more marked polar disposition of parts becomes evident. 
At one end, which, as subsequent development shows, becomes 
the anterior end of the animal, the cells are decidedly elongate, 
while more posteriorly they are still rounded. The nucleus 
in the elongate cells appears at the distal end, while, in the 
rounded posterior cells, it appears in the centre. 
Formation of the Yolk Columns. 
The elongate outer cells begin to assume a columnar form,, 
whose main body consists of an elongate mass of yolk cells 
with a peripherally placed nucleus (PL 13, figs. 14 and 15, 
e.r. y. e.). This is probably due to their increase in number, 
and consequent mutual pressure. The elongate character is 
gradually assumed by the other cells in a more posterior 
position, and ultimately all the outer cells assume the form 
of columns with peripheral nuclei. The last of these outer 
cells to assume the columnar form is a group of rounded 
cells at the extreme posterior end, from which the internal 
cells are proliferating. The internal cells are still some- 
what rounded, and ultimately completely fill the blasto- 
coele, so as to form a solid mass of cells. 
