Fkbeuaky 18, 1898.] 



SCIENCE. 



223 



out and take no further part in subsequent 

 divisions. The spindles that form after 

 this time are very small and resemble the 

 central spindle described by R. Hertwig for 

 other echinoderm-eggs. The experiment 

 shows that the additional sodium chloride 

 added to the sea water acts as a stimulus 

 on the nucleus, starting in it a series of 

 changes leading to a division and separa- 

 tion of the chromosomes. The effect lasts 

 through a long series of subsequent divis- 

 ions. The artificial asters, as long as pres- 

 ent, seem to act as centers towards which 

 the chromosomes move. The rays of the 

 astrospheres that come in contact with 

 the chromosomes change their structure 

 in very much the same way as do the rays 

 that form the spindle in the ordinary 

 karyokinetic figure. 



Centrosome and Sphere in the Fertilised Egg of 



Vnio. F. E. LiLLiE. 



Starting with the typical structure of 

 the aster in the metaphase of either ma- 

 turation spindle, viz. : A small centrosome 

 with the radiations inserted in it, and sur- 

 rounded by inner and outer strata of micro- 

 somes forming inner and outer spheres, it 

 was shown that by fusion of the stratum of 

 microsomes bounding the inner sphere and 

 by peripheral accumulation of its ground 

 substance, the inner sphere is converted 

 into a vesicle during the anaphase and tele- 

 phase of both maturation divisions. This 

 vesicle is now the central area of attach- 

 ment of the radiations ; and the centrosome 

 proper is attached to the wall of the sphere 

 by fibers, which are not part of the general 

 system of radiations. 



It was shown further that the central 

 spindle of the second maturation division is 

 formed within the inner sphere, and that 

 during the prophase the centrosomes in- 

 crease greatly in size and fragment into a 

 number of centrosome gramdes, one of which 

 remains as the centrosomes of the later 



stages (mother-star and later), while the 

 others form in part the stratum of micro- 

 somes bounding the inner sphere, and in 

 part become resolved into the ground sub- 

 stance of the inner sphere. 



Combining these results with those an- 

 nounced before the Society in the winter of 

 1896 (Science, V., 114, March 5, 1897), the 

 study of the maturation and fertilization of 

 the egg of Unio was stated to offer the fol- 

 lowing evidence against the theory of the 

 permanence and uniqueness of the centro- 

 some : 



1. A sperm amphiaster is formed, but it 

 disappears utterly at the time of the meta- 

 phase of the first maturation spindle. 



2. Entirely independently of the sperm 

 and egg asters, there arises in the egg of 

 Unio at the time of the metaphase of the sec- 

 ond maturation spindle an accessory aster, 

 in the center of which is a minute centro- 

 some. This centrosome divides and a 

 small amphiaster is formed, which entirely 

 disappears at the beginning of the tele- 

 phase. 



3. After the formation of the second po- 

 lar globule the egg centrosome goes the way 

 of its kind (i. e., disappears). 



4. The two cleavage centrosomes arise 

 independently of any of their predecessors, 

 and apparently separately. 



5. Fission products of the centrosomes 

 become cytomicrosomes. 



Thus the egg of Unio furnishes evidence, 

 in the first place, that the centrosomes are 

 not genetically continuous ; in the second 

 place, that a centrosome may arise de novo 

 (accessory aster) ; and, in the third place, 

 that products of division of the centrosome 

 may become other formed elements of the 

 cell. 



A somewhat fuller statement is to appear 

 in the Zoological Bulletin 



Tlie Fertilization of the Egg of Molgula Man- 

 hattensis. H. E. Crampton, Jr. 



