PROTOMITOSIS 191 



prophase is difficult to see, but the metaphase is very distinct and 

 shows the chromosomes very clearly on the equator of the spindle. 

 At the completion of the first division, which was held by Maire 

 and Tison to be a reducing division, there is a second division, 

 which is presumably of the same nature as the homotypic division 

 of the higher plants. 



This very curious process of protomitosis or cruciform division 

 has always been held to be the correct interpretation of the nuclear 

 divisions of these plants, and the various stages described by Cook 

 have been found by all the earlier workers. His description of the 

 process merely differs from the older investigators by the use of 

 more up-to-date methods of technique. A very different inter- 

 pretation of the nuclear phenomena exhibited by the Plasmodio- 

 phorales has, however, recently been put forward by Home. A 

 number of different genera were investigated, e.g., Spongospora, 

 Sorosphcera and Plasmodiophora. Home claimed that the somatic 

 divisions are typically mitotic in character, and that all the existing 

 accounts are entirely unsatisfactory. According to his description 

 the nucleus has a definite membrane, and viewed from a median 

 plane presents a configuration very similar to that of a wheel 

 with a centrally-disposed nucleolus and chromatin threads radiat- 

 ing from it. The same general type of structure was found in the 

 resting nuclei of all three genera. At the onset of division the reti- 

 culum breaks down into a spireme. At metaphase the chromo- 

 somes become united end-to-end to form a continuous band sur- 

 rounding the nucleolus, and this band then divides to form two 

 daughter bands, which finally break down to form the daughter 

 chromosomes at anaphase and telophase. The nucleolus is a pro- 

 minent structure of the nucleus, and persists throughout division. 

 It is these continuous bands of chromosomes which, according to 

 Home, have been misinterpreted by previous workers into the 

 well-known cruciform structure exhibited by these nuclei. 



After an unrestricted period of mitotic activity, the nuclei 

 cease to divide and become achromatic. Presumably this achro- 

 matic appearance of the nuclei corresponds to the akaryote stage 

 of other investigators. Home assumed that a fusion of coenocytic 

 growth forms takes place at some stage, and that this union in the 



