Nos. 451-452.] STUDIES ON THE PLAXT ( ELL. 593 



It is probable that the eggs of other conifers will be foiiiul to 

 present much the same protoplasmic structure and ac tivities as 

 those of the pine. Thus Murrill (: 00) describes for the hemlock 

 spruce (Tsuga) a vacuolar receptive spot and figures masses of 

 food material very much like the proteid vacuoles. The general 

 features of the egg of Cephalotaxus (Arnoldi, : 00a), Thuja 

 (Land, : 02), Podocarpus (Coker, :02), Taxodium (Cnker. 03) 

 have been recently described and those of Abies, l.arix and 

 Taxus are familiar from older writers but the j)ine remains as 

 the type of conifer in which the events of oogenesis are best 

 known as regards the details of protoplasmic activities. 



Besides the pine we have had some very complete investiga- 

 tions on cycads and Ginko (Hirase, '98, Ikeno, '98b and :oi, 

 Webber, :oi). In some respects these types and especially the 

 cycads seem to be the most favorable of all the gymnosperms 

 for the study of gametes and the processes of fertilization (to 

 be described in Section IV). The cytoplasm of the egg is com- 

 paratively homogeneous in structure so that the cell is relieved 

 from the complicated fibrous structure and proteid vacuoles 

 present in the pine. Ikeno ('98b) finds that the egg of Cycas 

 develops a crater like depression just before and at the time of 

 the fusion of the sperm thus presenting a rather highly special- 

 ized receptive spot. 



We know almost nothing of the detailed structure of the egg 

 in the Gnetales. Ephedra (Strasburger, '72) develops arche- 

 gonia much like those of other gymnosperms and we should not 

 expect their eggs to be materially different ( 1 t ils Ikit 



the conditions in Tumboa ( Welwitschia) are i^eculiar and apjjroach 

 more closely those of angiosperms where the egg nucleus is 

 scarcely differentiated from neighboring nuclei lying freely in 

 the protopla.sm at one end of the eml)ryo sac. The eggs of 

 Tumboa (Strasburger, '72) are merely cells of the prothallus that 



presents a further simplification or reduction since the female 

 nuclei lie freely in the protoplasm at one end of the embryo sac. 

 In Gnctum gnemon the lower half of the embryo sac is filled 

 with a tissue (Lotsy '99) but in several other species studied by 

 Karsten ('92, '93) no cell walls are found in the entire sac until 

 after fertilization. 



