THE GENERAL OUTLINE OF MITOSIS 13 



metaphase. This is not the case, however, in many 

 of the Protozoa, where what appear to be nucleoli 

 often persist through the entire mitotic cycle.* In 

 many cases nucleoli have been shown to be attached 

 to particular pairs of chromosomes (e.g. the sixth in 

 order of size in Maize) during prophase 40, 105 . 

 apparently the point of attachment is always one 

 of the constrictions referred to above. It has been 

 stated that in these cases nucleolar material contri- 

 butes to the growth of the chromosomes during 

 prophase, but the evidence for this is unconvincing. 



2. Prometaphase 



At the end of prophase the nuclear membrane 

 usually disappears. In many of the Protozoa, and 

 even in some higher forms, however, it persists and 

 the whole process of mitosis is intranuclear.* The 

 term prometaphase designates the period from the 

 dissolution of the nuclear membrane up to the end 

 of the process of spindle -formation. In ' intra- 

 nuclear mitosis ' there is no stage which can be 

 separately distinguished as prometaphase. 



The mode of origin of the spindle varies con- 

 siderably, but it is probably possible to reduce the 

 essential details to a common plan. In the simplest 

 cases it is formed (probably entirely out of nuclear 

 sap after the dissolution of the membrane) as separate 

 spindle elements corresponding in number to the 

 chromosomes. This is the type of spindle found 

 in the meiosis of some scale-insects '^ and in the 

 female meiotic divisions of Artemia salina^ the 

 Brine Shrimp (Fig. 12).^^ Usually these spindle ele- 

 ments fuse completely to form a single gelatinous 

 body in which the separate elements are no longer 

 visible (Fig. 2c, rf), but in the above cases they 

 remain distinct, and do not even converge towards 

 ' poles ' but end in fan-shaped expansions (Fig. 3a). 

 In these and a number of other cases no trace can 

 be found of centrosomes or asters, which leads one 



