334 PROCEEDINGS OF THE ACADEMY OF . [April, 
rule, and by observing the position of this furrow the first and second 
cleavage planes may be kept distinctly in mind until outwardly visible 
differential changes in the quadrants present other landmarks for orien- 
tation. 
ORIGIN OF GERM LAYERS. 
Segregation of the Ectoblast. 
By the next three divisions in which the four macromeres participate 
the entire ectoblast arises. 
First Quariet.—The spindles which precede the appearance of the 
first quartet of micromeres lie at first nearly radial, their prox- 
imal ends being distinctly higher than the distal. As a rule, all 
four spindles do not show the same stage of karyokinetic activity, 
though irregularities of this nature are not as yet greatly marked 
(fig. 9). As division proceeds they turn in a dexiotropic direction and 
with associated cytoplasmic constrictions four small cells are given 
off toward the animal pole (Pl. XXII, figs. 10,11). These, the first 
quartet of micromeres, are in size about one-fourth that of their 
parent macromeres. As they round out in shape they are pushed 
farther toward the right, and finally come to lie in the furrows to 
the right of the large cells from which they arose. With the com- 
pletion of cleavage the whole egg again takes on a decidedly rounded 
contour, the micromeres changing materially in shape, becoming 
more, flattened on their outer surfaces and sharp-angled below to 
fit the indentations between the macromeres (fig. 14). 
Second Quartet.—The second quartet arises leotropically, thus regu- 
larly alternating in direction of cleavage with the first. The derived 
micromeres are but slightly smaller than the underlying cells from 
which they arise and are pushed strongly toward the left as they are 
given off. By this movement the four cells of the first quartet are 
also carried somewhat to the left, though the rotation is not great. 
All the second quartet cells are alike in size, there being no sign of 
increase in D quadrant, as is the case with many Annelids and some 
Mollusks; nor is there marked difference in their time of origin, though 
in future cleavages of the egg irregularities in the time at which divi- 
sions occur in similar cells of the four quadrants become more and 
more marked. In cytoplasmic structure these cells appear to differ 
little from their parent macromeres, though probably they contain 
less yolk. Their ultimate position is opposite and beneath the divi- 
