MUSEUM OF COMPARATIVE ZOOLOGY. 247 
completed and the egg is in the 2-cell stage, this plane passes through a 
rosy pole. While this gap in observation is too important to be over- 
looked in studying the relation of the primitive plane of cleavage to 
the poles of the egg or the axis of the adult animal, enough has been 
observed to show that the first plane of cleavage passes through the 
pole of the egg adjacent to that part of the sac which is attached to 
the gonophore, if the rosy pole of the egg in the 2-cell stage and that 
of the egg in the gonophore are the same. We are able to identify 
a rosy pole in the egg, even into those post-segmented stages when 
the embryo begins to push out the two layers of the primitive hydro- 
phyllium on the surface of the yolk; and while we have not traced the 
continuity of this pigment in an egg in this stage with the segmented 
egg older than the 8-cell stage, the presumption is that the poles are 
the same in both cases. 
The primary furrow, p7., bending into the ovum on one side of the 
Agalma egg, causes many obscure or sharply defined folds on each side. 
Similar plications are also mentioned and figured by Metschnikoff* in 
Epibulia. The egg at this time as shown by Metschnikoff in the latter 
genus resembles the ova of Geryonia and the Ctenophora. 
As the groove on the animal pole deepens, changes in the external 
contour of the egg follow with great rapidity. I have timed the dura- 
tion of a few of these variations, and give camera drawings to illustrate 
their appearance at intervals of time. 
At 8h. 45 m. in the morning the indentation which marks the appear- 
ance of the primary cleavage furrow has just begun to appear. The egg 
at this stage is smaller than that just laid, but whether this diminution 
in size is due to the changes which result from the formation of the 
primary furrow or individual variation, we have no data by which to de- 
termine. The diameter of this egg in the plane connecting the pole 
where the furrow has taken place with the opposite is .30mm.; the 
longer diameter is .35mm. The profile of the egg, looking at it in 
a plane at right angles to the primary furrow, is oval or slightly notched 
at one pole. 
Fifteen minutes later, at nine o'clock A. m. ( Pl. I. fig. 7), the profile of 
the same egg in the same position has become still more heart-shaped, 
and the primary furrow has deepened to an amount greater than the 
radius of the egg. The depression forming the primary furrow almost 
girts the egg, extending over the surface for more than two thirds its 
circumference. 
* Op. cit., p. 40. 
