INTRODUCTION. 
487 
representative of the macrospore, that part of the ovule in which the embryo-sac 
arises (the riucellus) must be considered the equivalent of the macrosporangium. 
But, as in the ibrmation of the ovules of Monocotyledons and Dicotyledons, certain 
processes of development (the formation of the archegonia or ' corpuscula being 
no longer necessary, are suppressed, and the oosphere is immediately produced 
within the embryo-sac as the analogue of the macrospore, so also the production 
of the embryo-sac immediately from the tissue of the nucellus of the ovule is more 
direct. Its production is due to the increase in size of an inner cell of the nucellus 
which here represents the sporangium. But while even in the most highly developed 
Cryptogams the macrospore still becomes detached from the mother-plant, and the 
full developmejit of the prothallium takes place only after the dissemination of the 
spores, so that the embryo always arises in structures distinct from those of the 
mother-plant, the embryo-sac (or macrospore) of all Phanerogams remains, on 
the contrary, enclosed in the ovule, the endosperm in the embryo- sac, and the 
embryo in the endosperm. In this manner arises that structure peculiar to Phane- 
rogams, the Seed, the testa of which, the product of the envelopes of the ovule, 
closely invests both endosperm and embryo. The whole becomes separated from 
the mother-plant after the embryo has attained a certain very variable degree of 
development. Germination consists in the further development of the embryo at 
the expense of the endosperm. 
If, on the other hand, the microspores of Selaginella and Isoeles are compared 
with the pollen-grains of Phanerogams, a series of analogies is again seen which be- 
comes intelligible on comparing the intermediate phenomena presented by Gymno- 
sperms. Indications of the male prothallium and antheridium are indicated, as 
Millardet and Pfeffer have shown, by certain cell-divisions which may also be recog- 
nised in a simpler form in the pollen-grain of Gymnosperms and in a still simpler 
form in those of Angiosperms. Like the microspores, the pollen-grains contain the 
male fertilising substance, which, passing into the oosphere, causes it to develope the 
embryo ; but a great difference is displayed in the mode in which the fertihsing 
substance is conveyed. In Cryptogams the fertilising substance takes the form of 
antherozoids endowed with motion and adapted to force themselves, with the 
assistance of water, into the oosphere through the open neck of the archegonium. 
In Phanerogams, where the oosphere is enclosed in the embryo-sac and ovule, and 
in Angiosperms by the wall of the ovary in addition, such a conveyance of the 
fertilising substance would not serve the purpose intended; the pollen-grains are 
therefore themselves conveyed to the ovule by foreign agencies, such as the wind, 
mechanical contrivances in the flowers, and especially insects; and then germinating 
like spores, they emit their pollen-tubes, which, penetrating through the tissue of the 
ovule, finally reach the embryo-sac, and transmit the fertilising substance to the 
oosphere. The analogy of pollen-grains to spores becomes still more evident when 
we examine the mode of origin of both. The mass of tissue in which the pollen is 
formed, the pollen-sac, shows, not only in its morphological but also in its anatomical 
relationships, a striking resemblance to the sporangium of Vascular Cryptogams. 
As in the latter the spore-mother-cells are formed by the isolation of cells previously 
combined, so also are the mother-cells of the pollen ; and as the former themselves 
usually produce the spores by division into four, after previous indication of a 
