252 BULLETIN OF THE 



At 1 h. 45m. P.M. (fig. 21) the protoplasmic forces are still active in 

 sending out the rhizopodia, and the secondary cleavage plane (2 d. pL) 

 is not wholly formed, and at 1 h. 55 m. p. m. (fig. 22) the 4-cell stage 

 is practically complete, although here and there, as at the left of the 

 figure, a slight protoplasmic elevation can be seen. The second plane 

 of cleavage is practically formed. 



An hour and ten minutes, 2 h. 10 m. P. m. (fig. 23), after the 2-cell 

 stage we have an egg divided into four cells by two planes at right 

 angles to each other. None of these cells have a nucleus, and all are 

 still penetrated by the network of "cells" which we have already de- 

 scribed in the unsegmented ovum. The vertical plane passes through 

 a rosy region of the egg ; the opposite pole is more transparent. The 

 diameter of the egg on the first cleavage plane is a little less than 

 .50 mm.; on the opposite plane, about .45 mm. Although on the face 

 of the egg which is before us the ovum is divided into the 4-cell con- 

 dition, I have not been able to observe the opposite pole. Subsequent 

 stages seem to indicate that the secondary plane does not extend 

 wholly through it, but that at the opposite side there still remains 

 an undivided surface. Later changes in the general outlines of the 

 ovum lead me to suspect that the undivided part, either by growth 

 or protoplasmic extension, is of considerable size after the formation of 

 the 4-cell stage. 



Third Cleavar/e Ftirroiu. — The appearance of another cleavage farrow 

 on the same e^Q, the third which has been traced, was first noticed at 

 3h. 15m. P.M. (PI. II. fig. 3), two hours and thirty minutes after the 

 2-cell stage. In the mean time certain changes in the contour of the 

 egg which are not fully understood had taken place. At 3h. p. m. (PI. 

 II. fig. 1) the primary (Id. 2^1.) and secondary (2 d. pi.) planes of 

 cleavage, represented by the vertical and horizontal planes, occupy the 

 same relative position as formerly, and the right-hand cells are in the 

 main the same in contoui*. On the side of the left-hand cells, as fig- 

 x;rcd, away from the observer, has appeared a large undivided lobe (et-), 

 a little smaller than the original left-hand cell of the 2-cell stage. 

 The egg has probably been slightly rolled on its axis, by which the large 

 undivided lobe is turned into sight, whereas formerly it was concealed 

 behind the two left-hand cells of the 4-cell stage. I was not able to 

 observe satisfactorily the origin of this large lobe. The only explana- 

 tion which can at present be given to account for its existence is one 

 suggested above, that it is the bridge or connecting band which has not 

 been divided by the second cleavage furrow. If, however, its fate 



