able to answer the question whether the nucleolus in Zygnema, like 

 that of Spirogyra, possesses a membrane. 



A few nucleoli did not seem to correspond to the above descrip- 

 tion, although I cannot definitely state, that these were aberrant. 

 I have never met with very abnormal nucleoli such as Escoyez 

 observed in the resting nucleus so that the question occurs to me 

 whether such nucleoli do not arise in fixing, like the perinucleolar 

 cavity or whether perhaps they may be of a pathological nature. 



Karyohinesis. In investigating karyokinesis in Zygnema the first 

 question concerned the behaviour of the thread with tickened ends 

 in the nucleolus, for in Spirogyra crassa I was able to determine 

 witliout interruption the karyokinetic changes in the two nucleolar 

 threads. In Zygnema I have not succeeded in doing this. Observa- 

 tion in this oase must necessarily be so much more minute and is 

 accompanied by so many more difficulties, that after a few futile 

 attompts 1 was obliged to abandon the study of the changes in the 

 nucleolar thread. I am unable therefore to give any further infor- 

 mation about this important point. 



At the beginning of karyokinesis the nuclear network has a 

 somewhat coarser and looser structure; everywhere there arise by 

 aggregation portions which are much thickened, whilst the meshes 

 become wider. The nucleolus acquires an irregular shape and. seems 

 to dissolve completely. By further aggregation of the nuclear network 

 threads are formed, resembling strings of pearls. The nuclear-wall 

 is then still visible. In later stages it is dissolved and the network 

 has formed a number of short, thick corpuscles, which are connected 

 together by thin filaments. Meanwhile there arises from the cyto- 

 plasm gatheiicd round the nucleus a well developed nuclear spindle, 

 whose pointed poles extend to the chromatophores. The nuclear 

 network now moves back more and more into the equatorial plane, 

 so that finally there is in the centre of the nuclear spindle a flat 

 round disc surrounded by the spindle-fibres. This is the nuclear^ 

 plate. It is composed of a number of small bodies resembling short 

 thick pieces of thread or lumps which are joined to each other by 

 fine threads, or they may be intimately connected or completely united. 

 Their number cannot be determined. Clearly visible and well-formed 

 chromosomes, such as occur in some Spirogyra species to the number 

 of 12 or 6, are not found in Zygnema, but there is no great objec- 

 tion to calling the small, short bodies of the nuclear plate chromo- 

 somes, in agreement with the usual nomenclature. The mass 

 out of which the nuclear plate is composed appears noticeably 

 smaller than that of the network of the resting nucleus. 



