364 PROCEEDINGS OF THE ACADEMY OF [April, 
mainly concerned in the closure of the blastopore, though the deriva- 
tives of 2a”-2d” also bound the narrowing opening. Planorbis 
(Holmes) shows a very similar condition, with the exception that 2d” 
is crowded out. In Fiona all second quartet cells but a few at’|the 
anterior edge of the blastopore are excluded before the opening closes. 
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ORGANOGENY. 
The Velum. 
In its earlier stages the velum of Fiona is so ill-defined on the upper 
surface of the developing larva that its study has proved most diffi- 
cult, and though more time has been spent upon this region than any 
other portion of the developing organism the results have not been 
as satisfactory as could be wished. Living material would have been 
of great value, and the lack of it has been a source of much regret. 
After the breaking up of the cross the whole external surface of the 
gastrula, and particularly the anterior end, is characterized by cells of 
small and nearly equal size, among which there appear scarcely any 
cells whose size would give them prominence, or cell rows or distinctly 
marked groups. 
In the last stage described under the discussion of the develop- 
ment of the first quartet the area covered by this series of micromeres 
represents nearly the whole upper surface of the flattened gastrula (fig. 
75). The four arms of the cross are split transversely, while in the 
angles between them lie the four groups of turret cells, each group 
consisting of four cells of equal size. In axial relation the anterior and 
posterior arms correspond to the direction of the median plane, while 
the lateral are respectively right and left. The whole first quartet 
area is completely surrounded and separated from the third by deriva- 
tives of the second. By an increased growth of D quadrant of this 
series the apical pole and its surrounding area is moved forward in 
the direction of the blastopore, while at the same time growth of first 
. and second quartet elements in the neighborhood of the tip of the ante- 
rior arm of the cross causes that region to become raised, until some- 
what later the pointed anterior end so characteristic of many Opistho- 
branch larve is produced (figs. 78, 79, 96). The visible cause of the 
evagination of the ectoderm at this point may be found in the direc- 
tions taken by spindles of the dividing cells which produce it, as in 
most cases they are radially or diagonally direeted toward the point 
of greatest elevation. At this time the archenteron is roughly trian- 
gular in outline, the anterior point of the triangle being marked by 
