No. 4-] THE FIRST CLEAVAGE OF LEPAS. 175 



made at intervals of one minute. The vitelline membrane 

 occupied a fixed position with reference to lines on the slide 

 and on the drawing board. The line a-b marks the long axis 

 of the vitelline membrane, which before cleavage passed through 

 the animal pole of the ovum. The cleavage furrow appeared 

 in a plane passing through the animal pole and oblique to the 

 long axis of the ovum (Fig. i). As the cleavage furrow slowly 

 deepened, the plane of cleavage rotated until, at the time of 

 complete separation of the first two blastomeres, it was almost 

 always transverse to the original long axis of the ovum ; that 

 is, it occupied a position at right angles to its first position, 

 which was still clearly indicated by the unaltered form of the 

 vitelline membrane {a-b, Fig. 4). In some ova observed the 

 rotation was less, and at the close of the cleavage the plane of 

 separation was oblique to the original long axis of the ovum. 

 Such a condition would be well represented by Fig. 3, if the 

 separation of the blastomeres were there shown as complete. 



Along with this rotation of the cleavage plane there occurred 

 a shifting of the position of the second polar body, which was 

 in the cleavage furrow, and was followed to a position about 

 90° from the point of its formation (Figs. 1-4). Occasionally 

 the polar body became detached from the ovum and failed to 

 shift with the rotating furrow. This seems to explain the few 

 cases observed by Groom, upon which he based his view that 

 the first cleavage is usually formed transversely to the long 

 axis of the ovum, and does not pass through the animal pole. 

 Studies of the preserved material of several other species and 

 genera lead me to believe that such a rotation takes place also 

 in them. The polar bodies are formed in a position about 90° 

 from that finally occupied by the first cleavage plane. There 

 is reason for believing that this plane passes through the animal 

 pole, for the second polar body is found in the cleava'ge furrow. 

 The relations to the vitelline membrane are as in Z. anatifera. 

 These considerations make it very probable that in the case 

 of all Lepadidae and Balanidae, whose development has been 

 heretofore described, a rotation of the cleavage plane will be 

 found to account for the apparently equatorial position of the 

 first plane of cleavage. 



