no 



WHITMAN. • [Vol. I. 



moved through an arc of 45° from its place of origin. Had 

 the segments maintained their original positions, it is evident 

 that the arrangement would have been opposite, instead of alter- 

 nate. The direction of rotation,^ which is the same as that fol- 

 lowed by the hands of a watch, is determined by the mode of 

 origin of the segments. The segment bb, arising subsequently 

 to the segment xx, pushes the latter to the left, and the posi- 

 tions thus established for the first two segments predetermine 

 corresponding positions for the two last-formed segments (^a« and 

 cc). Such a point might at first sight appear too trivial for special 

 notice, but its importance becomes apparent in view of a ques- 

 tion, now attracting a good deal of attention, as to the relation 

 of the first two vertical cleavage-planes to the median plane of 

 the embryo. There is no danger of being too exact in regard 

 to such relations, and the egg of Clepsine furnishes a case in 

 which they can be determined with a certainty that precludes 

 all controversy. As the micromeres represented, originally, the 

 upper angles of the macromeres, it is evident that the planes of 

 their apposed faces, marked by the boundary lines i and 2, must 

 be regarded as parts of the cleavage-planes I and II, although, 

 in consequence of the rotation of the quarterfoil, the former now 

 form angles of about 45° with the latter. The boundary lines, 

 I and 2, present the form of a cross, the arms of which, if ex- 

 tended, would bisect each of the macromeres. That limb (2) 

 of the cross, which originally formed a part of the cleavage-line 

 II, now marks the median plane of the future embryo. Here 

 then is a case in which the median plane of the embryo is very 

 clearly defined, and in which it coincides with neither of the first 

 two cleavage-planes, but forms an angle of 45° with each of 

 them. 



The cleavages resulting in the eight-cell stage are comparable 

 with the first two meridional cleavages and the first equatorial 

 or horizontal cleavage in the eggs of amphibia and some 

 fishes. 



The further history of the quarterfoil of micromeres has not 

 been worked out with sufficient thoroughness to say positively 

 and precisely what part they play in forming the embryo ; but 

 it seems quite certain that they contribute to the formation of 



'A similar rotation has been described bj Hatscbek (No. 35, p. 6) in Eupomatus. 



