MUSEUM OF COMPARATIVE ZOOLOGY. 249 



This is the nearest approach which we have in the Agalma egg to a 

 " resting stage." 



Second Cleavage Furrow. — How much time intervenes after the forma- 

 tion of the first cleavage plane before signs of a second furrow appear, 

 has not been accurately observed. It is thought to be about thirty 

 minutes. In a stage of segmentation, not raised from that just de- 

 scribed, but like it also in the 2-celI stage, it was possible to follow the 

 whole progress of the growth of the second furrow. This egg was not 

 raised from those formerly described, but was picked out of the water, 

 and was observed in the 2-cell stage on the fourth day after the Agalma 

 was captured. At 1 p. m. (fig. 13) it showed the first trace of the 

 second cleavage furrow, and an hour later the egg had passed into the 

 4-cell stage. The changes of that hour are as follows as far as external 

 form goes. 



If we suppose this egg to be placed in such a position (fig. 13) that 

 the first plane of cleavage (1 cl.pl.) is vertical, there will be observed 

 on one side of this plane, viz. in the left-hand cell, a slight depression 

 or furrow {se.) indicated at first by a variation from a straight line which 

 the plane seen in profile seems to have. This depression is caused, by 

 the infolding of the surface of the egg at that point, and is the begin- 

 ning of the second cleavage furrow. The furrow is at first at right 

 angles to the primary furrow, and in its earliest condition one cell only 

 of the 2-cell stage is modified. 



At 1 h. 10 m. p. M. (se., PI. I. fig. 14) the growth of the furrow is 

 very slight. The depression has deepened, the chasm widened, and 

 folds similar to those described in the walls of the primary furrow have 

 been developed. 



At 1 h. 15 m. p. M. (PI. I. fig. 15) the second cleavage furrow {se.), 

 while extending itself and deepening in the left-hand cell, has appeared 

 also in the right-hai>d as well. It is now no longer placed at right 

 angles to the primary cleavage plane, but lies across it at an angle of 

 from G0° to 65°. A slight predominance in size of the left-hand end 

 of the furrow is shown in the figure. The diameter of the egg at right 

 angles to the first plane of cleavage is now about .60 mm. ; the shorter 

 diameter, about .45 mm. 



The growth of the egg in the next three minutes is important. At 

 1 h. 18m. P.M. (fig. 16) the second furrow has lengthened and deep- 

 ened, growing in such a way as to produce a certain twisting in the first 

 plane of cleavage. A contortion of the first cleavage plane, 1 cl. p)l-, is 

 brought about by the growth of the second furrow. In an egg seen 



