458 Mottier . — The Embryology of some Anomalous Dicotyledons . 
is quite willing to admit the probability of the seed-habit as having arisen 
independently in Gymnosperms and in the Angiosperms, but it is certainly 
difficult to understand how a structure like the embryo-sac, which is 
exactly alike in the vast majority of both groups of Angiosperms, could 
have arisen otherwise than from a common ancestor. The embryo-sac 
is certainly as deeply seated as any known structure, and if morphological 
characters have any great value in determining phylogeny, the embryo- 
sac tells no uncertain story. This may not be said with equal emphasis 
for the seed-habit or for the organs of an embryo. The embryo in the 
mature seed of Zamia, for example, bears a marked resemblance to 
that of a Dicotyledon, yet the difference in the early development of 
each is very great. Furthermore, it may be said that the first step toward 
the seed-habit was taken when the female prothallium of a Pteridophyte 
was retained within the macrospore, and the greatest progress toward seed 
formation found in any living Pteridophyte is the female prothallium with 
its enclosed embryo in the macrospore of Selaginella and Isoetes. Let 
it be understood, however, that the writer does not imply that the change 
from the habit now existing in Selaginella or Isoetes to an angiospermous 
seed has been a direct one, but conditions like these seem to indicate 
the probable line of development leading towards seed-formation. 
The embryo-sac is still our greatest stumbling-block, and, as stated 
above, if deep-seated morphological characters count for much. Monocotyle- 
dons and Dicotyledons have had a common ancestor, or their parent stocks 
were very closely related. Probably the majority of recent writers agree 
in the monophyletic theory of the origin of the Angiosperms, but they 
differ as to which of the two classes is the more primitive, and as to the 
relative value of the evidence brought forth in support of their respective 
views. 
Lyon (’01), in a well-written essay, argues in favour of the monophyletic 
theory, regarding the Monocotyledons as the more primitive stock from 
which the Dicotyledons have been derived. The monocotyledonous stock, 
according to his view, is derived from pteridophytic ancestors, the foot 
of the embryo of a Fern being homologous to the cotyledon. ‘ It (i. e. the 
foot) is to the pteridophyte embryo in a simple way what the cotyledon is 
to the embryo of a Monocotyledon, and is in fact to be considered as a more 
primitive type of cotyledon 3 (1. c., p. 66). The period of life of the sporo- 
phyte of Bryophytes and Pteridophytes within the calyptra is regarded 
as comparable to the intra-seminal life of the angiospermous embryo, and 
the single cotyledon of the monocotyledonous embryo is considered as 
having bifurcated to give rise to the two cotyledons in a manner similar to 
the bifurcation of the cotyledonary primordium of Nelumbo . Lyon’s point 
of view is well stated, and his theory is suggestive, but it seems to the writer 
that the facts do not indicate that the foot has undergone the morphological 
