208 



CYTOLOGY 



CHAP. 



massed together into a " chromidial net " which forms a ring round 

 the edge of the cell. The Arcella therefore now consists of a cell with 

 two nuclei (primary nuclei) and a chromidial net. 



At certain stages of the life history secondary nuclei are formed in 

 large numbers out of the chromidial net, by the aggregation of chromidia 

 into small masses. These secondary nuclei multiply by mitosis, and 

 ultimately may give rise either to asexual buds or to gametes. 



An unusual type of syngamy is sometimes observed in this animal, 

 involving fusion or mingling of chromidia (chromidiogamy, Swarczewsky) 

 rather than of formed nuclei. Two Arcellas, in which the primary nuclei 

 have degenerated and all the chromatin is in the form of finely scattered 

 chromidia, come together. Their cytoplasms — and hence the chromidia — 

 mingle together, and then separate again into the two individuals, each 



41. 



B 



Fig. 88. 



Arcella vulgaris. {A, B, after R. Hertwig, Festschr. Kupffer, 1899 ; C, after Swarczewsky, A.P.K., 1908.) 

 A, active phase, with two primary nuclei and a chromidial net ; B, degeneration of primary nuclei and 

 formation of secondary nuclei out of the chromidial net ; C, mitosis of a secondary nucleus. 



c, chromidia ; p.n., primary nucleus ; s.n., secondary uuclei. 



containing presumably a mixture of chromidia from both conjugants. 

 Out of these chromidia secondary nuclei are organized in a manner 

 similar to that described above. 



In Coccidium schubergi also, according to Schaudinn, the micro- 

 gamete nuclei are produced from that of the microgametocyte by the 

 passage of the chromatin of the latter out of the nucleus into the cytoplasm 

 in the form of chromidia which rise to the surface of the cell, and there 

 become aggregated into the microgamete nuclei. 



In Coccidium, Arcella, and still more definitely in Masiigella, therefore, 

 during certain periods of the life history the chromatin is aggregated 

 into a formed nucleus, and all the usual mechanism of mitosis is present 

 to ensure its accurate quantitative and qualitative partition among its 

 descendants. At other periods this methodical mode of nuclear division is 

 interrupted by a method of nuclear multiplication which affords no direct 



