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 



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

 vegetative nuclei of the myxamcebse ; 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 ; 

 1, 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 ' myxamcebse," amoaboid 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- 



