SYNGAMY AND SEX IN THE PROTOZOA 



149 



period of active reproduction. Chromidiogamy is a rare but very 

 interesting form of syngamy which, from the standpoint of general 

 notions with regard to the evolution of the nucleus, may be re- 

 garded as the most primitive type. It is known to occur also in 

 Difflugia (Zuelzer, 85), where also copulation of swarm-spores takes 

 place as an alternative method (p. 230). 



A case must now be considered in which the chromidia represent 

 vegetative, while the nuclei contain the generative, chromatin. 

 An example of this state of things is furnished by Plasmodiophora 

 brassicce, a well-known parasite of cabbages, turnips, etc., in which 



FIG. 76. Gamete-formation and syngamy in Plasmodiophora brassicce. A, Normal 

 vegetative nuclei of the myxamoebse ; B, C, extrusion of chromidia from 

 the nuclei ; D, division of the nuclei by karyokinesis (first reducing division) ; 

 E, nuclei after reduction ; F, formation of gametes which are fusing in pairs ; 

 G, spore (zygote) containing two nuclei, one of which is going through a 

 further reduction-division ; H, fusion of the two pronuclei within the spore ; 

 /, ripe spore with synkaryon and two centrioles. After Prowazek (127), 

 magnified about 2,250 diameters. 



it produces a disease known as " fingers and toes " (Kohlhernie). 

 According to the investigations of Prowazek (127) and others, 

 Plasmodiophora goes through a development which may be briefly 

 summarized as follows : At the end of the " vegetative " period of 

 growth and multiplication, there are found within the cells of the 

 infected plant a number of " myxamoebse," amoeboid individuals 

 (plasmodia) each with many nuclei containing distinct karyosomes 

 (Fig. 76, A). From the nuclei chromidia are given off into the cell, 

 and during this process the karyosomes disappear and centrosomes 

 make their appearance (Fig. 76, B, C). The chromidia are ab- 



