1904.] NATURAL SCIENCES OF PHILADELPHIA. 335 
this distinction be drawn, for until that time all quadrants appear 
identical, though doubtless cytoplasmic and nuclear differentiation is 
present. As a result of this similarity of all the quadrants the figures, 
until the appearance of the mesentoderm cell, have of necessity been 
labelled arbitrarily. Of course, even in the two-cell stage lateral may 
be distinguished from terminal areas, for by following succeeding 
cleavages and marking the relation which the lower polar furrow bears 
to the first cleavage plane and the later relation of both to the median 
plane of the embryo, it can be determined that the first cleavage plane 
is obliquely transverse to the median plane. But not until a later 
period does posterior become distinguishable from anterior end. 
In the formation by first cleavage of two cells of equal size, Fiona 
agrees with a large number of Mollusks and Annelids, among the former 
of which may be mentioned Ischnochiton (Heath, 1899), Neritina 
(Blochmann, 1881), Crepidula (Conklin, 1897), Ercolania (Trinchese, 
1880), Tethys (Viguier, 1898), Planorbis (Rabl, 1879, and Holmes, 
1900), Limazx (Kofoid, 1895, and Meissenheimer, 1896), and among the 
latter Lepidonotus (Mead, 1897) and Podarke (Treadwell, 1901). 
Unequal cleavage appears to occur as commonly as equal among 
Opisthobranchs, examples of which are Acera (Langerhans, 1873), 
Aplysia (Blochmann, 1883; Carazzi, 1900), Umbrella (Heymons, 1893) 
and Philine (Guiart, 1901). 
SECOND CLEAVAGE. 
The second cleavage results in four cells of approximately equal 
size. The spindles which precede it lie at right angles to the first 
cleavage spindle. and nearly parallel to each other, the left end of each, 
however, being slightly higher than the right, showing the leotrophic 
character of the division. As cleavage proceeds this tendency becomes 
more marked, the upper or left-hand cells (A and C) lying higher than 
the right (Band D). In consequence of this the second cleavage planes 
do not meet in a line at the vegetative pole, but a portion of the original 
first cleavage plane unites them in the ventral polar furrow (“Quer- 
furche”’ or “‘ Brechungslinie’’), the cells B and D being in contact below, 
while A and C never meet at the lower pole. At the upper pole no fur- 
row is present in Fiona, the four cells all joining in a common central 
point. As is the rule among Annelids and Mollusks in which the 
second cleavage is leotropic, the ventral polar furrow taken in connec- 
tion with the first cleavage plane, bends to the right when viewed from 
the animal pole, and, vice versa, it turns to the left if considered as a 
part of the second cleavage plane. Fiona is no exception to the above 
