MUSEUM OF COMPARATIVE ZOOLOGY. 25a 
resembles that of the protoplasmic bridge of the primary furrow, my 
explanation is probably erroneous. 
The large lobe is the point of origin of a new cleavage furrow, which 
I have called the tertiary or third cleavage furrow (3 cl. pl.). In the 
general structure and mode of origin the third cleavage furrow bears a 
striking likeness to the primary and secondary. It forms at right angles 
to the direction of the second furrow and parallel with a part of the 
first furrow in the large undivided lobe on the left-hand side. At 3h. 
5m. P.M. (PIII. fig. 2) the tertiary furrow had not begun to appear ; 
but ten minutes after, at 3h. 15m. p.m. (PI. II. fig. 3), it had reached 
a considerable size. Like the primary and secondary furrows, the walls 
of the tertiary are formed by an infolding of the surface of the ovum, 
and have the characteristic sharply defined folds and plications already 
mentioned. 
Figures of the egg at 3h. 20m. p.m. (PI. II. fig.4), and at 3h. 
25 m. p. M. (fig. 5), are introduced in order to show the progress of the 
growth of the tertiary furrow in the division of the large undivided 
lobe on the left-hand side of the egg. At 3h. 30m. P.M. (fig. 6) two 
hours and a half after the formation of the first cleavage furrow, the 
tertiary furrow has divided this lobe horizontally into two smaller cells. 
The portion of the tertiary plane which bisects the large lobe is, like 
the primary and secondary, perpendicular to the plane of the paper on 
which the egg is figured. The two axes of the egg, a vertical, which is 
the original cleavage plane, and the horizontal, the secondary plane, are 
easily distinguished, and at one end of the tertiary furrow, now almost 
completely closed in, there is figured a marked protoplasmic elevation. 
This stage is a 6-cell stage, composed of the four cells which have 
already been mentioned and the two additional which have just formed. 
The tertiary furrow was the third furrow observed, but I suspect that 
between the secondary and tertiary (by my nomenclature) the large lobe 
which I have represented as divided by this furrow was constricted from 
the two left-hand cells by another, whose growth was not observed. 
Morula. 
The complications in the growth of the ovum after the stage last 
mentioned make it very difficult to follow the birth of new segment 
spheres or cleavage planes. The last stage of the egg in which the 
course of the original cleavage can be traced with any certainty is at 
3h. 45m. p.M. (PI. II. fig. 7), or two hours and three quarters after the 
