FERTILIZATION. from asexual spores, and il is not surprising that they occasionally germinate as asexual spores. In the simplest cases of fertilization the gametes are similar, so thai there is mi distinc- tion of sex. In this ease the process is often called 'conjugation,' and plants which exhibit it are known as 'isogamous' plants. Almost all plants, however, are 'heterogamous;' that is, the pairing gametes have heco so different that a distinction of sex is plainly evident, in heterog- amous plants the male gamete is known as the 'sperm,' and the female gamete as tin- 'egg,' and fertilization in its restricted sense is the fusion of sperm and egg. Along with the differentiation of gametes has occurred the differentiation of sex- organs (gametangia), those developing sperms being called 'antheridia,' those developing eggs being called 'oogonia' or 'archegonia.' The result of fertilization is the formation of a spore which is in all plants technically the 'oospore,' or fer- tilized egg. To distinguish the result of conjuga- tion from that of ordinary fertilization, the name 'zygospore' or 'z}'gote' is commonly substituted for 'oospore.' Among seed-plants a single free egg is developed within the enibro-sac, and with- in the pollen-grain certain cells are developed which are called 'male cells,' and which are sperms in function. One of the chief features in connection with the process of fertilization is the method by which the pairing gametes are brought together. In conjugation the two gametes have similar powers, and in the majority of cases are motile 557 FERTILIZATION. sperm cea sea in be a motili - ■! i- ordina- rily transferred from the pollen-grain in which it i- produced to i he egg ly m i he 'pollen tube.' This tube in gj mnospei me (pines, etc.) penetrates thi I i he o> ule FERTILIZATION. 1, sex-organs of Yaucheria; 2, conjugation of two ciliated gametes; 3. a.large egg surrounded by numerous sperms: 4, autheridial tube entering r^ ,,f rvstopus; •">, sperms entering arehegonium of a liverwort; 6, fusing cells. bodies, usually swimming by means of cilia. In some cases in the lower forms the gametes are Brought together by the development of a tube which bridges the space between the sex-organs. In heterogamous plants the sperm is always the active gamete, and finds its way to the egg. which remains quiescent. Among the higher algae, mosses, and ferns the sperm is, as a rule, a free- swimming ciliated cell, and is attracted to the oogonium or arehegonium by various chemical substances which are secreted in connection with the female sex-organ. One of the distinguishing marks of the seed-plants, however, is that the DOUBLE FERTILIZATION IN LILV. .1. Fusion of sperm and egg; b, fusion of sperm with two polar nuclei. lying above the embryo-sac, and in the angio- sperms (flowering plants) it passes through the style, enters tin- ovary, and finds it-- waj to the ovule. It is an interesting fact that in the Cycadacese (q.v.) ciliated sperms have recently been discovered, which indicates that the old free-swimming habit of the sperm had not been entirely abandoned with the introduction of pol- len-tubes. While the passage of the sperm to the egg may be included under fertilization, the real act of fertilization consists in the fusion of the two naked cells. .lust what happens in this fusion may be illustrated by the process of fertilization in the seed-plants. Each gamete is a naked cell consisting of a nucleus about which (litre is organized cytoplasm. In the sperm the nucleus is very prominent, and the cytoplasm relatively scanty. In the egg the nucleus is also prominent. but the cytoplasm is abundant, and contains a rich supply of food reserve. In the seed-plants it has been observed that the pollen-tube approaches the egg and discharges a male cell into tin- cyto- plasm. The nucleus of the male cell then slips out of its cytoplasm as out of a sheath, and moves through the cytoplasm of the egg toward its nucleus. The male cytoplasm lias thus been left behind in the egg-cytoplasm, and it is not clear that the fusion of the two holds any im- portant relation to the act of fertilization. In any event, the mosi evident fact in fertilization is the approach of the male nucleus to the female nucleus, and the fusion of the two. .lust what this fusion involves, and how complete it is. is not clear. The nuclei are exceedingly complex structures, and just how far the corn p tiding structures of the two nuclei fuse in this process is very uncertain. In certain seed-plants, as in gvnmosperms, if has been observed thai the two sets of chromosomes, which are thus brought to- gether by the fusion of the two nuclei, do not fuse for some time, and in some cases for several
