1906] CHAMBERLAIN—OVULE OF DIOON 343 
the living genus with the thinnest membrane could be regarded as the 
farthest removed from its ancestry, and the genus with the thickest 
membrane would represent most nearly the ancestral condition. In 
spite of the uncertainty of such suppositions, it seems reasonable to 
regard a thick membrane as a primitive character, and while not 
conclusive evidence in itself, it deserves to be considered with other 
features in any discussion of the phylogeny of the cycads. 
It would be interesting to know the thickness of the megaspore 
membranes of the other cycads at various stages in the life history. 
THomson (21) found that in Cycas revoluta the membrane is slightly 
more than 5 in thickness at the stage when cell division is beginning 
in the endosperm. At a somewhat later stage the same writer found 
the membrane of Stangeria paradoxa to be 4.5 » in thickness, and the 
same thickness is given for the membrane of mature seeds of Zamia 
integrijolia; while in Ceratozamia longifolia the membrane is 4.54 
in thickness at a stage when the archegonia have been formed. In 
Dioon imbricatum,? before the appearance of archegonia, the thick- 
ness was 3.8. Ihave measured the membrane in the above genera, 
except Stangeria, and find only such slight variations from THoMsoN’s 
figures as may be accounted for by slight differences in stages of 
development. The extreme thickness of the membrane in the germ- 
inating seed of Dioon so much exceeds any of THoMSON’s measure- 
ments that it would be interesting to know whether the membranes 
of other gymnosperms increase so much in thickness when the seeds 
germinate. The megaspore membrane of Dioon is as thick as that 
of any cycad, and consequently, so far as this one character is con- 
cerned, Dioon is as primitive as any member of the group. 
Development of the endosperm.—Early in November, about two 
months after pollination, the endosperm has become cellular through- 
out. If one might hazard a guess at previous stages, the November 
Condition looks as if the free nuclear condition had been succeeded 
by the formation of very large cells which had been divided repeatedly. 
At a stage when the archegonium initial is first distinguishable, the 
peripheral cells of the endosperm about half way between the arche- 
gonial and chalazal regions present the appearance shown in fig. 19. 
is form is described by Mr1QueEt, in his Prodromus Systematis asl 
ets under Dioon edule Lindl., as 8 imbricatum, and is based upon leaf characters 
