244 Rickett.—Fertilization in Sphaerocarpos. 
of cases division of the wall cells does begin at the time that the polar caps 
begin to appear. Fifty such cases were counted ; while only six showed 
a division in the wall of the venter at a time when the formation of polar 
caps had not begun. Twenty-seven eggs had formed their polar caps with¬ 
out any sign of division in the wall of the venter, but in many of these cases 
the polar caps were formed in advance of the usual time. It seems probable, 
therefore, that the division of the cells of the venter, if not stimulated by the 
changes just described in the egg cytoplasm, is at least usually coincident 
with them. The division is complete, and the venter consists of two layers 
or cells, in all cases observed in which the chromosomes are organized in the 
sexual nuclei. 
Sixth Phase , 
This phase includes the final steps in the organization of the chromo¬ 
somes of both nuclei, the disappearance of the nuclear membranes, the splitting 
of the chromosomes, and the division of the latter to form the two daughter 
nuclei of the first embryonic division. Next to the penetration of the egg 
by the antherozoid, this is the most difficult of all the phases to observe. 
This fact is evident from a glance at Table I. Only eight cases are listed 
under this phase. Of these, four show the nuclear membranes intact, but 
enclosing fully organized chromosomes (Fig. 16). 
The organization of chromosomes in the male nucleus was not seen. No 
transition stages were obtained between the granular condition last described 
and that of those nuclei which contained a clearly defined spireme or chromo¬ 
somes lying within the nuclear membrane. The change is probably, therefore, 
very rapid; and the suggestion is inevitable that the spireme and chromo¬ 
somes are organized during the contraction period of the nucleus—much as 
the gemini of the heterotypic prophases are organized in synizesis during the 
reduction division. 
One case was observed in which the chromatic substance of the male 
nucleus was in the form of an apparently continuous spireme, coiled within 
the nuclear membrane. This is shown in Fig. 15. In another case, shown 
in Fig. 16, instead of a spireme there were eight distinct chromosomes, the 
regular gametophytic number for Sphaerocarpos (Allen, 1919 ). One of these 
is very small, and only by good fortune in the making of the section did it 
escape being masked by one of the larger ones. This is undoubtedly the 
Y-chromosome described by Allen. It is not a cross-sectional view of a long- 
D 
chromosome, since it disappears completely from view with a slight change 
in focus. 
The female nuclei in these cases had not quite completed the organi¬ 
zation of their chromosomes ; the outlines of the latter were not quite so 
sharp, nor was their consistency apparently so uniform, as was the case with 
those of the male nuclei. Hence it would seem that, although the female 
