1907] CAROTHERS—GINKGO BILOBA 11g 
the development of the lower of the two mother cells in a single ovule 
(fig.6). COULTER (3) suggests that this early development of the 
lowest cell may be the result of a favorable position in regard to 
nutriment coming up through the chalazal tissue. 
The functional spore enlarges rapidly, its nucleus being held near 
the upper end of the cell by a large vacuole below. The presence 
of this vacuole in the spore suggests the probability of the parietal 
position of the free nuclei of the embryo sac from the very beginning. 
The appearance of a vacuole and the consequent parietal place- 
ment of the two nuclei resulting from the first division of the spore 
nucleus is reported by LAND in Ephedra trijurca (8); but in Ginkgo 
the vacuole is present even before the formation of the first gameto- 
phytic spindle, being in fact the same vacuole that was in the mother 
cell. 
PRoTHALLIUM.—Free nuclear division —Ovules of two weeks later 
are somewhat larger, and the embryo sacs contain free nuclei (fig. 20), 
the number ranging from 16 to 64. At this stage the sac is very thin, 
staining no more deeply than does an ordinary cell wall, and the 
plasmic layer lining it is very delicate. The nuclei are well separated 
and each has a conspicuous nucleolus. They are lenticular in side 
view and circular in face view, and average 13.5 # in long diameter 
by 8.14 in short diameter. In these early stages the division of 
the free nuclei is simultaneous, although each of several sacs with 
64 spindles shows the latter in several different phases (fig. 21). 
This is of interest, inasmuch as later free nuclear division proceeds 
irregularly, nuclei in a single sac showing all conditions through the 
various stages of karyokinesis and resting (fig. 24.) At first the nuclei 
of such sacs are definitely placed, so that although all stages are to be 
seen in the sac, the nuclei in any given portion are in the same stage. 
Later the sacs show nuclei in all stages grouped without regard to 
condition. 
Free nuclear division continues from the second week in May 
until the first week in July, the whole ovule meanwhile enlarging and 
the embryo sac growing rapidly in both size and thickness. The 
protoplasm becomes granular and the free nuclei divide so rapidly 
that they decrease in size (figs. 22, 24). By the first of July the cyto- 
plasm has become very granular (fig. 23), and the number of nuclei 
