i9i 2] YAMANOUCHI—CUTLERIA 459 



of the two gametes comes into direct contact. The male gamete can 

 be observed for a short time as a protuberance at the periphery 

 of the female gamete (fig. 95); however, sooner or later the pro- 

 tuberance is entirely absorbed or leveled down to the surface of the 

 now spherical female gamete. The male nucleus with 24 distinct 

 chromosomes proceeds toward the resting female nucleus (fig. 96); 

 it then proceeds still further (fig. 97) until it is close to the female 

 nucleus (figs. 98, 99). The male nucleus at this stage is surrounded 

 by a clear zone, which possibly means that the nucleus is carrying 

 with it a part of the male cytoplasm or a part of the female cyto- 

 plasm which had been lying in the path of the male nucleus. The 

 male nucleus now becomes closely applied to the female nucleus 

 (fig. 100). The chromatin of the male nucleus becomes closely 

 aggregated (fig. 101), and then begins to be finely alveola ted (fig. 

 102), and finally becomes dispersed through the female nucleus 

 as irregular knots (fig. 103). Later the fusion nucleus contains 

 chromatin knots of various sizes and shapes, together with delicate, 

 irregular, discontinuous fibrils. A nucleolus which was present 

 in the female nucleus before the union still persists (fig. 104). The 

 fusion nucleus thus passes into a completely resting condition, in 

 which chromatin of male and female origin cannot be distinguished 

 from each other. 



Returning to the previous stage, at the time of the union both 

 gametes are surrounded by plasma membranes only. At the time 

 of the entrance of the male nucleus into the female cytoplasm the 

 outermost layer of the plasma membrane is observed to change into 

 cell wall (fig. 96), which is very thin at first, but gradually increases 

 m thickness as fusion progresses, and is well organized by the time 

 the fusion nucleus reaches the resting condition. 



The first segmentation division of the sporelings of the fertilized 

 gametes takes place 20-24 hours after the union of the gametes. 

 In prophase there appear 48 chromosomes, all of which seem alike 

 both in size and shape (fig. 106), and a single nucleolus persists. 

 At metaphase the chromosomes are arranged at the equatorial 

 plate and two centrosome-like structures appear at the poles 

 (fig. 107). A comparison of the prophase and metaphase shows 

 that the volume of the nucleus is considerably diminished at the 



