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, 2^f'; 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 8 h. 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 fx-om 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 .30 mm.; the 

 longer diameter is .35 mm. 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. ( PI. 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. ciL, p. 40. 



