1904.] NATURAL SCIENCES OF PHILADELPHIA. 355 
additional landmark for distinguishing anterior from posterior quad- 
rants (Pl. XXIV, fig. 25). The larger cells of the posterior quadrants, 
3c’ and 3d!, divide next; the spindle in 3c! being dexiotropic and 
alternating, that of 3d! leotropic and non-alternating; and this lack 
of alternation in one of the large cells of the third quartet, taken in 
connection with the regular alternation of the similar cell on the oppo- 
site side of the posterior region of the egg, establishes the first bilat- 
eral cleavage (Pl. X XV, figs. 31, 32,34). Both upper and lower cells 
of A and B quadrants are the next third quartet elements to divide, 
the direction in all cases being dexiotropic or in some instances nearly 
meridional (figs. 37, 40,41). The lower cells, 3a? and 3b’, always divide 
before the upper, 3a and 3b!, and in all cases cleavage is equal, a group 
of four similar cells arising in each of the two anterior quadrants. 
In the posterior quadrants cleavage occurs next in 3d”, 3d", 3c” and 
dc". It will be remembered that when these cells were formed it was 
through a leotropic and non-alternating division of 3d‘ and a dexio- 
tropic and alternating division of 3c’, thus producing a bilateral cleay- 
age of similar cells of opposite sides. Now the cells 3c" and 3c” 
again divide dexiotropically, thus showing lack of alternation, while 
-3d" and 3d” again exhibit distinct leotropic cleavage and a second 
failure to alternate. Thus arise in each posterior quadrant two very 
small cells, 3c", 3c! and 3d”, 3d!”, lying below the large ones, 3c™, 
3c", 3d! and 3d™ (Pl. XXVI, figs. 48, 44, 45, 47). After these 
cleavages about eighty blastomeres are present (figs. 67, etc.). When 
this number has increased to slightly over a hundred, 3a”, 3a”, 3b”! 
and 3b”, each gives off a small cell toward the vegetative pole by cleay- 
ages which appear horizontal (Pl. XX VII, figs. 57, 59), and these divi- 
sions are followed by equal and probably horizontal cleavages in the 
posterior quadrants of the large cells, 3c™, 3d‘ and 3c™! and 3d™, 
the former pair always dividing before the latter (figs. ‘61, 66), so that. 
each posterior group contains seven cells, of which three are small 
and lie nearest the blastopore, being bounded externally by four large 
cells, 3e'tt, 112, 1211 1212 and 3d), 112, 1211, 1212 respectively. 
The history of the third quartet of Fiona thus far given adds another 
to the number of Mollusks in which it has been found that bilateral 
cleavages first appear in the posterior quadrant, and more particularly 
in the cells of the third quartet. 
The initial divisions of these cells in Umbrella appear from Heymons’ 
description to be nearly radial, but his figures show that in the case of 
3c and 3d cleavage is leotropic. The lower products of these cleavages 
are all smaller than the upper, in which they parallel only the posterior 
