No. 3] THALASSEMA AND ZIRPHAEA. 595 



XXXI, Fig. 11). Owing to this intimate connection, the dis- 

 carded nuclear skein is, I believe, shifted along with the spindle, 

 and there is, therefore, no means of orientation such as was 

 found by Hertwig ('78) in Asteracantliion, where the spindle is 

 figured as leaving behind the remains of the nucleus as it rises 

 to the surface. 



These considerations involve an important problem touching 

 the polarity of the egg in relation to the cleavage-planes. We 

 have seen (p. 588) that the eccentricity of the germinal vesicle, 

 and hence that of the primary egg-axis, are to be referred to 

 the position of the young egg in the cluster, the nucleus shift- 

 ing toward the free side. If the rotation of the spindle is such 

 as to bring it to always lie in the egg-axis, then the position of 

 the polar bodies and the first cleavage-plane will, of course, 

 depend upon the position of the young egg in the cluster. But 

 another possibility is open. The outer aster remaining station- 

 ary, the spindle might rotate about it as a pivot just sufficient 

 to bring the spindle-axis in an egg radius, in which case the 

 position of the spindle, and hence that of the polar bodies and 

 cleavage-plane, would be independent of the early egg-axis, and 

 would vary according as the asters appeared in one or another 

 of numerous possible positions about the germinal vesicle. 



A rotation of the first maturation-spindle — generally through 

 90 degrees of arc — has been described in numerous forms. 

 Thus Sobotta ('95) observed it in the mouse, Boveri ('87) in 

 Ascaris, Weismann and Ischikawa ('88) in Arteniia and Eupa- 

 guriis, Brauer ('92) in Branchipus, etc. It appears, however, 

 from the accounts of these authors, that it takes place in these 

 forms much later than in Thalasscma. Other observers find 

 no evidence of rotation and believe the spindle to reach the 

 surface by a motion of translation only, in the direction of its 

 axis {cf. Korschelt, '95 ; Wheeler, '95 ; Kostanecki and Wier- 

 zejski, '96). 



As the spindle rises, the outer aster soon encounters the 

 membrane. The more outwardly directed rays in consequence 

 steadily shorten, while the more lateral ones are deflected and 

 curved backward — a phenomenon also well shown in Physa 

 (Kostanecki and Wierzejski, '96). In the definitive position 



