78 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



egg as a whole through an arc of 90 within the vitelline membrane. 

 The long axis of the two-cell stage is, therefore, at right angles to the 

 chief axis, which has rotated 90 from its original position of coincidence 

 with the long axis of the vitelline membrane. The chief axis, which is 

 the longer axis of the unsegmented egg, becomes the shorter axis of the 

 two-cell stage. An examination of Figures 1-16, which represent a 

 series of camera lucida drawings made at intervals during cleavage, will 

 make clear the changes in form and position which the egg of Lepas 

 undergoes in the course of the first cleavage. 



In a preceding chapter it has been shown that, after the formation 

 of the second polar cell and at about the time of the union of the pro- 

 nuclei, the yolk becomes partially separated from the protoplasm and 

 becomes aggregated at the vegetative pole of the egg (Figs. 2-6, 18-20). 

 Shortly afterwards it is shifted to one side of the polar area (Figs. 7, 8) ; 

 this is the first indication that the egg is rapidly approaching cleavage. 

 Soon a wide shallow groove appears, passing obliquely around the ovum 

 from the animal pole (Fig. 8). The furrow rapidly deepens and the 

 forming cells become spheroidal, causing the ovum to elongate perpen- 

 dicularly to the plane of cleavage (Figs. 9, 10). The ovum as a whole 

 at the same time gradually rotates within the vitelline membrane (Figs. 

 10-15) ; consequently the plane of cleavage rotates until, at the comple- 

 tion of cleavage, the furrow is usually transverse to the long axis of the 

 vitelline membrane, still unchanged in form ; that is, the cleavage furrow 

 occupies a plane almost at right angles to that in which it at first ap- 

 peared relative to the vitelline membrane (compare Figs. 8 and 15). 

 These facts explain the copflict between the conclusions of earlier obser- 

 vers and the generally accepted idea that the first cleavage is meridional 

 in the ova of nearly all animals. 



The figures show that the second polar cell continues to lie in the 

 cleavage furrow, and consequently has retained a fixed position with 

 reference to the egg during its rotation within the vitelline membrane. 



In some ova the rotation is through less than a quadrant, so that at 

 the close of the first cleavage the plane of division is more or less oblique 

 to the long axis of the vitelline membrane. In examining living ova 

 taken at random, many oblique cleavage furrows are noticed, but con- 

 tinuous observation usually shows that the obliquity is the result of 

 preparation for the second cleavage. Accordingly, it may be stated as 

 a general rule that at the close of the first cleavage of the ova of Lepas 

 the cleavage plane is transverse to the long axis of the vitelline mem- 

 brane, and that only in comparatively few cases is it markedly oblique. 



