34 THE ASTER IN ARTIFICIAL PARTHENOGENESIS 



The movement of the egg nucleus is possibly also a case in point. 

 As long as the egg nucleus is beyond the confines of the aster it is sta- 

 tionary. As soon, however, as the extending aster reaches it, the 

 nucleus begins travelling toward the sphere in which it finally lies 

 close beside the sperm nucleus. The existence of a centripetal cur- 

 rent may be inferred also from the following experiment. In an 

 egg one may occasionally see one or more oil-like droplets 2 to 4 

 microns in diameter. If one of these droplets be pushed by the 

 needle from the liquid cytoplasm into the periphery of the aster the 

 droplet will move along the rays toward the center. 



In view of the above observations it is highly probable that the 

 liquid which accumulates in the center of the aster streams into it 

 from all sides during the jellying of the cytoplasm. It is this stream- 

 ing which probably occasions the innumerable radiations characteris- 

 tic of the aster. After the aster has attained its full size the radia- 

 tions begin to fade from view as the jelly state reverts to a more 

 fluid one. The liquid of the central sphere does not mix with the 

 fluid cytoplasm but separates into two areas, one at each pole of 

 the mitotic figure of the dividing nucleus. Astral radiations now 

 appear about the two areas as the egg cytoplasm jellies again with the 

 formation of two jellied masses instead of one, as heretofore. These 

 grow at the expense of the fluid cytoplasm until all of the cytoplasm 

 of the egg is taken up into two bodies, the two blastomeres of the 

 segmenting egg. 



During the rapidly succeeding cleavages of the egg there is always 

 a cap of liquid on the nucleus of each blastomere. With each mitosis 

 this liquid flows around the nucleus to accumulate in two areas at 

 the poles of the mitotic figure. These areas are periodically aug- 

 mented during the formation of an aster and the ensuing jellying 

 process. 



There is every evidence^ that the mechanism of cell division de- 

 pends upon a readiness of the cytoplasm to pass from a liquid to a 



^ Heilbrunn, L. V., Studies in artificial parthenogenesis. II. Physical changes 

 in the egg oi Arbacia, Biol. Bull., 1915, xxix, 149; An experimental study of cell 

 division. I. The physical changes which determine the appearance of the spindle 

 in sea-urchin eggs. J.Exp. Zool., 1920, xxx, 211; Chambers, R., Changes in proto- 

 plasmic consistency and their relation to cell division, /. Gen. Physiol., 1919, 

 ii, 49. 



