74 MCMURRICH. [Vol. XI. 



cells A and B being the products of the original cell A, and C 

 and D the result of the division of the original C of Fig. 5. 

 It would be interesting to know definitely whether these two 

 arrangements represent two modes of segmentation ; or whether 

 the cases in which the inclination of the axes of the two pairs 

 to each other is comparatively slight, are simply stages in 

 rotation through an angle of 45° of each of the two pairs of 

 cells formed by a typical meridional division. I did not suc- 

 ceed in obtaining ova in the spindle stage preceding this divi- 

 sion, and therefore cannot answer this question positively, but 

 from the circumstantial evidence at my disposal I am inclined 

 to believe that there is a rotation, but also that in a number of 

 ova it remains incomplete. 



In the cases in which the rotation has reached its fullest ex- 

 tent some interesting features are brought to light when a 

 comparison is made with the corresponding stages of other ova. 

 The existence of a cross-furrow has recently attracted consid- 

 erable attention in connection with its significance as indicating 

 a " spiral " form of cleavage. The rotation of the ^g% oijaera 

 may be regarded as an extreme form of "spiral" cleavage, and 

 it becomes of interest to note the relationships of the furrows 

 which separate the various cells. In this particular egg no cross- 

 furrows exist in reality on account of the nature of the seg- 

 mentation, but it is not difficult to imagine what their relations 

 w^ould be were the cleavage holoblastic. Practically the same 

 condition would obtain which has been described by Ludwig 

 ('82) for the ovum of Asterina in the four-celled stage, and this 

 condition may be regarded as due to the rotation of each pair 

 of cells through an angle of 45°, the arrangement thus being 

 essentially similar to that described by Wilson ("92, p. 452) for 

 Nereis. In the figures of Jaera the conditions appear to be 

 slightly different, but this is due to the slightly different posi- 

 tion in which the ^gg is drawn, a position which may be imi- 

 tated by rotating the Nereis ^^'g through 45°, when A and B 

 will lie in one plane and C, D, in another plane at right angles 

 to the former. 



In Jaera, then, we have to deal with a rotation of each of the 

 two pairs of cells through an angle of 45°, and the arrangement 



