PASTO) M. M. Mercaur 
shorter and thicker and, after from half an hour to an hour, the 
two bodies are almost completely fused. Frequently one can distin- 
guish for quite a time upon the zygote a slight protuberance bearing 
a few weak cilia of the microgamete type, thus marking very 
clearly the point of fusion, which may be in any region of the body 
(Fig. 182, Pl. XXIII, cf. O. caudata, Figs. 265, 268, Pl. XXVII). 
I have seen all possible stages in the copulation, have three times 
followed the whole process from the first contact (observed) to the 
complete fusion, and have many times followed the process through 
the earlier or later or middle period and have then killed and stain- 
ed the animals in order to observe more closely the nuclear phe- 
nomena. 
The macrogametes may have either one or two nuclei (Pl. X XIII). 
The microgamete has always but one. When the macrogamete is 
uninucleated the nucleus may be in the resting stage (Fig. 164, Pl. 
XXIII) or in mitosis (Figs. 167, 169, 170, 182, Pl. XXIII); similarly 
the two nuclei in the binucleated females may be either resting 
(Figs. 171—173, 178. Pl. XXIII) or in mitosis (Fig. 180, Pl. XXIID. 
I have never seen the nucleus of the microgamete in mitosis before 
the complete fusion of the two bodies. 
When the macrogamete has but one nucleus this unites ob- 
liquely end to end with the nucleus from the microgamete (Figs. 
187—196, Pl. XXIV). The two nuclei apply themselves closely, and 
ultimately the double membrane between them breaks down (Figs. 
191—193), sometimes first in the middle sometimes first at one edge. 
If the macrogamete has two nuclei, conditions become more 
complicated. If the two female nuclei are in the resting condition, 
the male nucleus may fuse with either one. In one instance of 
copulation of this type I have watched the entrance of the male 
nucleus and have seen it fuse with the posterior nucleus of the 
macrogamete, the resultant fusion nucleus being considerable larger 
than the other nucleus in the same zygote (Figs. 174—177, Pl. XXIII). - 
I had a single acetic-carmine preparation which suggested that the 
male nucleus had entered and had passed by the posterior nucleus 
of the binucleated macrogamete and was just uniting with its anterior 
nucleus (Fig. 186, Pl. XXIV). Sometimes one sees the male nucleus 
lying between the two female nuclei, generally nearer to the anterior 
one (Figs. 197—200, Pl. XXIV). In one lot of fine material from 
an eighty-eight hour infection I found that over fifty of the bi- 
nucleated forms had one nucleus (almost always the anterior one) 
much the larger. In some of these larger nuclei it was easy to see 
