46 - NATURAL SCIENCE. Jan.. 



centrosome has lost its function, it retains its original size and 

 shape, and the fact that it surrounds itself with an archoplasmic 

 sphere, points to some of its original power being still left. Further- 

 more, there can be little doubt that, whether we homologise 

 the centrosome with the micro- or macro-nucleus, it was origi- 

 nally derived from the single primitive nucleus. Where it appears to 

 come out of the nucleus before, and to go back again after, karyokinetic 

 division, it is merely a recapitulation of its past history. A point in 

 advance has been gained by those cells where the centrosome 

 remains permanently outside the nucleus. 



No mention has been hitherto made of the remarkable obser- 

 vations of Herr Lauterborn on living diatoms, an account of which 

 appears elsewhere in this number of Natural Science (p. 7). His 

 results are so at variance with all previous known facts of karyo- 

 kinetic division, that at present it seems safer to draw no conclusions 

 until more confirmatory evidence has been obtained. Besides, 

 the mechanics of the processes which he describes are completely 

 unintelligible. 



The spindle. — What has been said with regard to the nuclear or 

 non-nuclear nature of the centrosome applies equally well to the 

 spindle. It appears to be formed by the activity of the centrosomes. 

 Where the latter remain permanently in the cytoplasm, the spindle 

 would appear to be made up of strands of protoplasm resembling 

 those which make up the archoplasmic spheres appearing between 

 the two daughter centrosomes. Where the centrosome has an intra- 

 nuclear origin, the spindle is undoubtedl}' formed from the achromatic 

 portion of the nucleus. 



Furthermore, two kinds of spindle must be distinguished, viz. : — 

 (i) The so-called "central" spindle; (2) the spindle which is made 

 up of two " half-spindles." An instance where the former obtains is 

 that taken as a typical case of karyokinesis. The second kind of 

 spindle is shown in Fig, g. It will be seen that it is discontinuous in 

 the equatorial plane, the fibres merely extending from the archoplasmic 

 rings to the chromosomes. The function also of the two seems to be 

 different. The two half-spindles appear to exert a pulling on the 

 chromosomes, whereas the central spindle is probably a groundwork 

 in which the chromosomes move by their own activity. Van Beneden 

 derives the spindle from both the cytoplasm and the achromatic 

 network of the nucleus. 



It would, however, seem probable that, where the spindle is derived 

 from the protoplasm of the cell (as it appears to be so in those cases 

 where the centrosomes are permanently extra-nuclear), it takes the 

 form of two half-spindles ; but where it is formed from the achro- 

 matic network in the nucleus, it is of the " central " type. This 

 point has not yet received sufficient attention. It may, moreover, be 

 urged against the assumption of two different kinds of spindle, that 

 the supposed existence of the central spindle is due to a wrong con- 



