702 SEXUALITY 



behavior: one, the stationary gamete nucleus, remains in the conjugant 

 that produces it; the other, the migratory gamete nucleus, passes into the 

 mate and unites with the stationary gamete nucleus located in that ani- 

 mal. As a rule, the gamete nuclei are morphologically indistinguishable; 

 but in some species differences in size and form have been reported. The 

 most extreme example of morphologically different gamete nuclei is in 

 Cycloposthii/m (Dogiel, 1925). The spindle resulting in the formation 

 of the gamete nuclei is heteropolar: one pole, destined to produce the 



^, 



Figure 167. Conjugation in Cycloposthiiim bipalmatum, showing the sperm-like mi- 

 gratory pronuclei differing from the spherical stationary pronuclei. (After Dogiel.) 



stationary gamete nucleus, is larger and rounder than the other smaller 

 and more pointed pole, destined to yield the migratory gamete nucleus. 

 The latter arises from the anterior pole of the spindle and develops a 

 long tail-like appendage at the proximal end, and a small, pointed distal 

 end, functional in piercing the cuticle in its passage from one mate into 

 the other (Fig. 167). In other ciliates, lesser differences between the 

 gamete nuclei have been observed: slight differences in size in Didinium 

 (Prandtl, 1906), in Paramecium caudatum (Calkins and Cull, 1907), 

 and in P. midtimkronucleatum (Landis, 1925). Calkins and Cull 

 (1907) concluded that the two gamete nuclei in P. caudatum differ in 

 their chromatin content, as a consequence of transverse chromosomal 

 division at the nuclear division which gives rise to them. In most ciliates, 

 however, no morphological differences between the two gamete nuclei 

 have been observed. 



