MUSEUM OF COMPARATIVE ZOOLOGY. 401 



vesicle. He traced its origin from the union of two nuclear structures, 

 which make their appearance at the opposite poles of the egg, and, after 

 attaining their characteristic features, migrate to its centre. 



After the egg has remained some time in a homogeneous condition, 

 these two structures simultaneously make their appearance as small clear 

 spots close under the surface at each pole. As no difference is recog- 

 nized between them, the further account is the same for both. At first 

 irregular in shape, this spot gradually enlarges and at the same time 

 becomes more nearly circular in outline, until, in the course of half an 

 hour, it has attained its full size and spherical form. It is homogeneous 

 and less refractive than the surrounding protoplasm, from which, although 

 sharply marked off, it is not separated by a membrane. It is a cavity in 

 the protoplasm filled with a substance probably fluid, as may be fairly 

 inferred from the rapid motion of the nucleoli observed later in its his- 

 tory. After a little time there appear within this homogeneous struc- 

 ture from one to five nucleoli, the size of which is generally inversely 

 proportional to their number. Just how they arise Auerbach is unable to 

 say. At first faint, they become darker, and then larger. If numerous, 

 they do not all appear at once, but one after the other in intervals vary- 

 ing from half a minute to a few minutes, and at poi«nts remote from each 

 other. 



These two thus fully formed nuclear structures now begin a slow mi- 

 gratory motion toward the centre of the cell, where they finally meet. 

 Meanwhile they suffer no change of form, but the nucleoli within them 

 often exhibit comparatively rapid changes of position. The migration is 

 gradually accelerated. Each nucleus leaves in its " wake " an indication 

 of the course it has pursued, in that the region traversed is less granular 

 than neighboring portions of the protoplasm. The cause of the motion 

 of the nucleoli the author is unable to explain.* The migration of the nu- 

 clei cannot have its cause in any power of motion inhering in the nucleus 

 itself, nor are centres of attraction discoverable ; in fact, any explanation 

 which presumes the protoplasm to be passive can hardly be accepted, 

 since its passive resistance to the motion of the nucleus would cause the 

 latter to become flattened in the direction of the motion. In short, it is 

 the contractile power of the protoplasm which forces onward the passive 

 nucleus, and the clear " wake " is rather the cause than the effect of this 

 migratory operation. The activity of the protoplasm also finds expres- 



* Subsequently (p. 247), he ventures the suggestion that it may be due (in case 

 the nuclei increase in size during their migration) to fine streams of fluid (Saftstrom- 

 chen) which must make their way from the protoplasm into the nuclear cavity. 



VOL. VI. — NO. 12. 26 



