LIFE HISTORY OF PRORODON GRISKUS. 313 



times, the latter case in Bursaria truncatella, before the differ- 

 entiation of the new micro-nucleus and the new macro-nucleus 

 occurs. In Prorodon griseus there is a single division. The 

 conjugation nucleus immediately after its formation, enlarges 

 somewhat and divides equally. The eight chromosomes are 

 distinct and can be co'unted without difficulty. The two cleavage 

 nuclei when first formed are quite similar in size and structure 

 (Figs. 24, 25). The two daughter cleavage nuclei often remain 

 connected by a delicate drawn out fiber, as shown in Fig. 26. 

 One of the daughter cleavage nuclei decreases somewhat in size 

 and produces the micro-nucleus. The other enlarges and be- 

 comes differentiated into the new macro-nucleus (Figs. 25, 26). 

 The different stages in the formation of the new macro-nucleus 

 are represented in Figs. 26 to 30. The chromatin network of the 

 macro-nucleus at first is quite uniform or about the same texture 

 throughout. The chromatin later, becomes quite distinct (con- 

 centrated) in the center, with a less dense spireme around its 

 border (Figs. 27, 28). The dark center of chromatin enlarges 

 until the nucleus is more or less homogeneous throughout (Figs. 

 2 9> 3)- The new macro-nucleus at first is spherical, but later 

 may assume different shapes as indicated above. The conjugants 

 become separated during the formation of the new nuclei. 

 According to Enriques ('08), in Chilodon uncinatus there is a 

 single division of the conjugation nucleus and that the two 

 daughter cleavage nuclei become differentiated into the new 

 macro and the new micro-nucleus respectively. 



THE OLD MACRONUCLEUS. 



The macronucleus of the free swimming forms, as indicated 

 above, is rather variable, ranging in shape from a circular form to 

 an elongated condition, or at times it is somewhat curved. 

 During early conjugation the nucleus when elongated shortens 

 slightly and frequently divides into two or more parts (Figs. 16 

 and 17). But in most cases it does not divide at all (Figs. 20 and 

 21). The chromatin becomes more and more condensed and 

 stains as a dark homogeneous body. The border of the dark 

 staining nucleus fades out, leaving a dark center within a clear 

 boundary (Figs. 23 and 25). Finally the dark center fragments 



