14 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
for other forms than Callidina russeola, whatever may be the conditions 
in that species. 
3. ORIENTATION OF THE DEVELOPING EMBRYO. 
The first cleavage plane is transverse to the long axis of the egg, and 
divides it into two unequal parts (Figure 6). The plane passes through 
the place where the polar cell was formed ; the smaller cell includes, 
therefore, that end of the egg nearest to which the polar cell is located. 
As previously stated, this is also the more pointed end of the egg, when 
any difference in the two ends is distinguishable. 
The second cleavage is approximately at right angles to the first, and 
nearly in the long axis of the egg. It also passes through the region 
where the polar cell was formed. 
As previously stated, the oval or ellipsoidal form of the egg is re- 
taimed throughout the early development. This form is independent ot 
the precise arrangement of the material of which the egg is composed. It 
is as if the egg substance were enclosed in a rigid mould of oval or ellip- 
soidal form. Within this mould the (fluid?) contents may shift their 
position widely, without influencing the form of tho mould. Thus, at the 
first cleavage, the material of the smaller blastomere occupies all of one 
end of the egg. In the ten-cell stage (Plate 3, Figs. 20-25) the same 
form is still preserved as if in a rigid cast, but the material which 
previously formed the smaller of the first two blastomeres has shifted 
from the end to one side of the egg. It therefore is necessary to have 
some term by which to designate the two ends of this constant form, as 
distinguished from the shifting blastomeres themselves. I shall hence- 
forth speak of that end of the egg at which lies the smaller cell in the 
two-cell stage as the micromere end of the egg, while the opposite region, 
where the larger blastomere lies, will be called the macromere end. 
These terms refer to the form of the egg, without regard to the shifting 
contents. 
The orientation which I shall adopt for the egg itself is similar to 
that used by Wilson, Heymons, Conklin, Lillie, Kofoid, and other re- 
cent workers on cell lineage. The region where the polar cell is formed, 
and which afterward lies opposite the blastopore, will be called the ant- 
mal pole ; it marks the dorsal surface. The opposite point is the vege- 
tative pole, marking the ventral surface, — the position of the future blas- 
topore. Dorsad signifies always toward the animal pole, or place where 
the polar cell was formed; ventrad, in the opposite direction, toward 
