12 INVERTEBRATE MORPHOLOGY. 



scattered in the form of a chromatic reticulum through the 

 substance of the nucleus, which thus passes again into the 

 resting stage, developing a new nuclear membrane. 



Our knowledge of many of the details of karyokinesis is yet imperfect, 

 and especially is this the case with regard to the mode in which the cen- 

 trosome exerts its influence. It has been regarded as a simple centre of 

 attraction, similar to the pole of a magnet, but the spindle-fibres seem to 

 be more than passive in the phenomena. A comparison of the centrosome 

 with an aggregation of microsomes has already been referred to, and if 

 this idea be extended some light may be thrown upon the spindle-fibres. 

 They would then naturally be regarded as reticular fibres, i.e. fibres of 

 plastin to which a certain amount of contractility and extensibility may be 

 ascribed. During the earlier stages of karyokinesis their extensibility is 

 more manifest, and extending into the nucleus they compress its chromatic 

 substance, the contractility manifesting itself later and determining the 

 migration of the chromatin loops or chromosomes towards the poles of the 

 nucleus. Furthermore, since the linin reticulum of the nucleus is probably 

 continuous with the plastin reticulum of the cytoplasm, it is conceivable 

 that the activities of the centrosomes may call out in it changes of contrac- 

 tion or extension which may suffice to bring about the characteristic skein 

 formation of the chromatin and the subsequent fragmentation of the skein 

 into the chromosomes, as well as the formation of the connective fibres in 

 later stages, the intermediate bodies upon these being regarded as micro- 

 somes. These views, which have been but recently suggested, require -con- 

 firmation, however ; if true they afford a new basis from which to attack the 

 problems involved in the phenomena of karyokinesis, and even at present 

 throw no little light upon the structural details associated with the process. 

 It must be mentioned, however, that certain recent observations have been 

 held to prove that the centrosome has a nuclear origin, and for the present 

 the important question of its significance must be considered as open. 



LITERATURE. 



0. Hertwig. Die Zelle und die Oewebe. Jena, 1892. 



W. Flemming. Zellsubstanz, Kern und Zetttheilung. Leipzig, 1882. 



0. Biitschli. Untersuchungen uber mikroskopische Schdume und das Proto- 



plitsma. Leipzig, 1892. 



C. Rabl. Ueber Zdltheilung. Morpliolog. Jalirbuch, x. 1884. 

 G. Platner. Beitrdye zur Kenntniss der Zelle und Hirer Theilung. Archiv fur 



mikrosk. Anatomie, XXXIII. 1889. 



M. Heidenhain. Uber Kern und Protoplasma. Leipzig, 1892. 

 H. P. Johnson. Amitosis in the Embryonal Envelopes of the Scorpion. Bulletin 



of the Museum of Comp. Zoology, xxn. 1892. 



