SEX IN INFUSORIA 223 



(PI. 17, fig. 2). The muscular diaphragm which is usually seen 

 in neuters and macroconjugants becomes obliterated. Finally, 

 there is a difference in the shape of the macronucleus. In the 

 macroconjugants and neuters the macronucleus resembles 

 a lemon. In the microconjugants (PI. 17, fig. 12) it is more 

 elongated, being at the same time rounded at both ends. Both 

 the micro- and macroconjugants differ from the neuters in 

 having only one contractile vacuole instead of two. 



Taken altogether, the differences between the conjugants are 

 so great that the individual members of a pair could be con- 

 sidered as belonging to different species, were they not found 

 in a state of conjugation. 



The conjugants adhere one to another with their anterior 

 ends, diverging at an acute angle from the point of conjunction. 

 I could follow on my slides all phases of the nuclear changes 

 characteristic of a typical conjugation. A cross-fertilization 

 takes place — both individuals interchanging their migratory 

 nuclei. Furthermore, I have found conjugating pairs with 

 a syncaryon in both partners, and a large number of excon- 

 jugants of both kinds. The latter show different stages in the 

 reconstruction of a normal nuclear apparatus. 



At first we find in the exconjugants two pronuclei sur- 

 rounded by a plasmatic halo (described by Prandtl in 

 Didinium, and present, I believe, in most of the Infusoria). 

 The old macronucleus persists in the microconjugant after 

 separation from its partner, while in the macroconjugant it 

 is already dissolved at the moment of disjunction. The double 

 syncaryon (PI. 17, figs. 3 and 4) forms the first division spindle, 

 and the reconstruction of the nuclear apparatus is very simple, 

 following the type represented by Chilodon. The first 

 division spindle by its fission gives rise to a pair of 

 nuclei which become respectively a new macro- and micro- 

 nucleus. An interesting feature of this division is its hetero- 

 polarity — from the diaster stage onwards. One of the polar 

 swellings of the dividing syncaryon is smaller and stains more 

 intensely with nuclear stains than the other one. It ultimately 

 becomes still more condensed, and develops into a micro- 



