634 FERTILIZATION 



present techniques some of the finer structural changes which occur 

 within the chromatin mass. 



Since Prandtl ( 1906) first noted a difi^erence in size between the wan- 

 dering and the stationary pronuclei of Didlnium nasutum, slight differ- 

 ences have been reported in a number of other cases. Calkins and Cull 

 (1907) showed that this is due to a heteropolar third division, in 

 Paramecium caudatum. Maupas (1889) vvas the first to record a struc- 

 tural difi^erence between pronuclei, when he observed the area of dense 

 cytoplasm in front of the migrating pronucleus of Euplotes patella. The 

 most striking dimorphism appears in Cycloposthiu77i, according to 

 Dogiel (1925). The wandering pronucleus is spermatozoon-like in hav- 

 ing an elongated tail. All these differences between pronuclei must be 

 cytoplasmic in origin, for the nuclei, as has been pointed out, are 

 genetically identical, if our present concepts are correct. 



STAGE E, MIGRATION OF PRONUCLEI AND FERTILIZATION 



Migration of the wandering nucleus occurs synchronously in the two 

 conjugants, so that they generally pass each other in the cytoplasmic 

 bridge which joins the two conjugants. In Cycloposthium (Dogiel, 

 1925), the spermatozoon-like male pronucleus passes out of the mouth 

 of the parent body and into the mouth of the recipient, by way of the 

 juxtaposed peristomal cavities (Fig. 167). 



In Euplotes patella (Turner, 1930), the wandering pronucleus breaks 

 out of the left anterior tip of the parent body, which is pressed into the 

 peristomal field of its mate (see p. 620, above), passes backward be- 

 tween the appressed conjugants, and finally enters the cytostomal area of 

 the recipient. Both pronuclei form spindles as the male approaches the 

 female, and four chromosomes can be seen in each. As the pronuclei 

 touch, their membranes dissolve and the two groups of four chromo- 

 somes mingle, as fertilization is completed and the diploid condition 

 restored (Fig. 151 D). In Chilodonella, MacDougall (1925) shows 

 that the two haploid chromosomes are visible throughout the migration 

 period (Fig. 152), but lose their identity soon after fertilization. 



The appearance of the pronuclei at the time of union varies with the 

 species. In a number of ciliates, they are in the form of a spindle similar 

 to those of Euplotes, although few show chromosomes. In other ciliates, 

 the pronuclei are spherical and vesicular at the time of union. In still 

 others, intermediate conditions have been reported. 



