432 J. A. MURRAY, 



what later stage (only oiie centrosome in the section). Here, in ad- 

 dition the centrosome lies in a depression in the nuclear membrane. 

 Fig. 14 represents a more exaggerated form of- the same condition, 

 the nuclear membrane being greatly crumpled and the whole nucleus 

 as it were compressed between the two radiations. 



The nuclear membrane now disappears (in Arion much earlier, 

 Fig. 22), and the typical amphiaster is formed (Figs. 11, 15, 23). 

 The chromosomes now divide in the equatorial plane, and the rings 

 become drawn out into ellipses, the long, attenuated, almost parallel 

 limbs of which may persist for some time. 



The centrosomes have steadily increased in size, reaching their 

 greatest development in the amphiaster. The appearance of the poles 

 in preparations stained with Bordeaux red and iron-alum haeraatoxylin 

 varies with the extent to which the differentiation is carried. If 

 only so far that the protoplasmic reticulum, while perfectly sharp, 

 remains of a deep violet colour we get the appearance depicted in 

 Fig. 10. The radiations end centrally in a large, dark, sharply de- 

 marcated sphere. If the process of decolourisation be continued, the 

 black sphere can be progressively diminished in size till only a 

 minute granule remains. The centrosome itself, i. e. the large spheri- 

 cal body, retains only the preliminary stain (Fig. 9 and 11). The 

 centriole ("Centralkorn") in the preparation from which these figures 

 are taken, is not spherical but appears drawn out into a point towards 

 the spindle fibres. A horizontal extension of the centriole indicates 

 the beginning of its bipartition, a process which is completed ■ by the 

 time the mitosis has reached its metaphase. If preparations stained 

 with iron-alum haematoxylin alone, be decolourised to a degree cor- 

 responding to Fig. 9 and 11 the spindle fibres thin off suddenly be- 

 fore reaching the centriole at a distance corresponding to the outer 

 limit of the centrosome (cf. Lee, 1. c. p. 238). If the differentiation 

 be pushed too far the centriole also is decolourised ^). 



If Figs. 8, 9 and 14 be examined carefully it will be seen that 

 the radiations end centrally in a point at some distance from the 

 nuclear membrane (as noted previously by Zimmermann, 1. c). This 

 distance increases with the progress of the centrosomes towards the 

 poles and in Fig. 14 (representing a condition probably not constant). 



1) This behaviour of the centrosomes corresponds exactly with 

 that in the egg of Ascaris megalocephala according to the researches 

 of Mr. E. TüRST of the Würzburg Zoological Institute. 



