1907] CAROTHERS—GINKGO BILOBA 125 
ovoidal, then ellipsoidal, of glandular cells encroaching upon and 
absorbing the surrounding nucellar tissue, the whole being enclosed 
in an enlarging cavity whose wall is made of elongating dividing cells. 
Although at first densely granular and non-vacuolate (jigs. 5, 6), the 
absorbing cells soon become vacuolate (fig. 18), then multinucleate 
(fig. 22). Whether these nuclei are the result of direct or indirect 
division is not known. That the former is probably the case is sug- 
gested by the occurrence of such division in the absorbing cells of 
other sporangia. Moreover, spindles are seen in early stages only, 
and in those cases a plate is always present. 
Until the fourth week in June the spongy tissue cells show great 
activity (fig. 22) in encroaching upon and absorbing the adjacent 
tissues, but soon afterward they themselves show signs of being 
absorbed by the enlarging prothallium (fig. 24), the latter being still 
in the free nuclear stage, but having formed the enclosing wall. 
By the time the radial walls are formed and centripetal growth has 
brought the tissues one-third of the distance across the sac, the spongy 
tissue cells have been absorbed (jig. 25), their remains being only a 
thin mass of collapsed and heavily staining walls. The embryo sac 
now lies against the undifferentiated nucellar tissue, separated from 
the living cells of the latter only by this mass of dead cells which have 
given up their substance. When the prothallium has become a mass 
of tissue, it is quite near the surface of the nucellus, instead of far 
below it as at first. Most of the upper part of the nucellus has been 
absorbed, and the nucellar beak, at first so conspicuous, has collapsed 
(fig. 34). 
Integument 
The tissue of the single, thick integument is homogeneous at first, 
but by the last of May it is differentiated into three distinct tissues 
(fig. 40): an outer layer of large thin-walled cells with many mucilage- 
filled cavities; a middle layer of small, isodiametric cells; and an 
inner region of large, very thin-walled cells loosely held together. 
This inner layer is further differentiated into an outer layer having 
cells transversely elongated, and an inner layer having cells vertically 
elongated. This innermost layer of vertically elongated, delicate- 
walled cells appears only in the free portion of the integument, but the 
