17 



Tlic nuclear plate of Zyr/neraa cruciatum is not ring-shaped. Miss 

 Merriman and Escoyez believe that tlie^' have seen annular nuclear- 

 plates in Zygnema, but I think this in a visual delusion. When a 

 nuclear-plate is seen edge-ways it may appear as if it were a ring, 

 but if looked at afterwards from the side, as is possible in using 

 the chromic acid method, all doubt immediately vanishes. I came to 

 the same conclusion with Closterium '), in which Lauterborn ^) had 

 described a ring-shaped nuclear-plate. 



Tlie nuclear-plate divides by longitudinal splitting into two halves 

 which separate. At first their structure becomes more dense. When, 

 by the use of chromic acid, they have been isolated and fall over, 

 they somewhat resemble round discs which appear spotted in conse- 

 quence of local differences in density. These halves of the nuclear- 

 plate develop into daughter-nuclei which acquire a membrane and 

 consequently show a sharp outline. It is difticult to say when the 

 nuclear membrane reappears. The dense structure of the halves of 

 the nuclear plate again gives place to a looser one and finally there 

 is again spread out within the nuclear-membiane a fine network, 

 which resembles that of the resting nucleus. During the entire process 

 of karyokinesis the nuclear network forms a coherent whole. When 

 the structure becomes looser again, the nucleoli also quickly appear. 

 At first there can be distinguished in the network many small masses 

 more or less globular and irregular, which gradually unite into 

 several larger masses and finally form one single spherical mass in 

 the centre of the nucleus. This representation of the origin of the 

 nucleolus differs very much from that given by Escoiez, but it 

 agrees with what has been observed in Spirogyra where also many 

 nucleoli flow together into a single one. 



There is a further point to be noted concerning the development 

 and position of the daughter nuclei in Zygnema cruciatum. In Spiro- 

 gyra and other cases the halves of the nuclear plate take up a 

 position near the poles of the spindle where they develop into 

 daughter-nuclei. In Zygnema cruciatum the development takes place 

 earlier. Before the poles have been reached, the daughter-nuclei 

 have already a membrane and consequently show a sharp outline, 

 whilbt the nucleoli have already united into several larger masses. 

 The spindle-fibres lie immediately against the daughter-nuclei. Between 

 the halves of the nuclear plate, the spindle greatly increases in 



1) G. VAN WissELiNGH, Ueber Kernstruktur und Kernteilung bei Closterium, Beih. 

 zum Bot. Gentralbl., Bd. XXIV a912), Abt. I. p. 429. 



2) R, Lauterborn, Unteisuchungen über Bau, Kernteilung und Bewegung der 

 Dialomeen, 1896. Fig. 68. 



2 



Proceedings Royal Acad. Amsterdam. Vol. XVI. 



