820 DE. H. R. W. PEAESON" ON THE MOEPHOLOGY OF 



which characterizes the earlier stages, and tend to become a^^regated more or less 



the chalazal end of the sac (fig. 9). At this stage the emhrvo-sac does not show the 



I vr 



" biscuit-like ' shape described and figured for a slightly later stage of G, Gnemon *. 

 Nor, in fact, does it eventually assume this form. As the nuclei further increase in 

 number, the aggregation at the chalazal end becomes more marked, and occupies 

 approximately the lower half of the sac ; the cytoplasm of this region also becomes 

 much less vacuolated than in the micropylar half. It is clear, therefore, that the 

 differentiation of the sac into micropylar (fertile) and chalazal (sterile) parts is 



lizable at least as early in Gnetum as in Welwitschia. Meantime the nucellar 

 tissue immediately beneath the chalazal end of the sac commences to form what Coulter 

 has called the " pavement tissue " f , a fairly regular^tissue in which the cells are more 

 or less strictly arranged in longitudinal rows (PI. 31. fig. 13). 



The next important change in the embryo-sac is the blocking-out of the cytoplasm of 

 the lower half into compartments. At the time of its occurrence the sac seems to be 

 in a particularly delicate state, and more liable to contraction than at any other period 

 of its history — a characteristic of the Welwitschia sac at a corresponding stage in its 

 development %. The first stage of septation— such as that figured for Welwitschia § 

 has not been seen. The youngest stage found (fig. 12), probably slightly contracted, 

 shows that each compartment possesses a definite limiting membrane which, however, 

 does not yet react to cellulose stains 1|, So far, then, the septation of the lower end of 

 the sac presents many similarities witli that of Welwitschia. The outlines of each 

 compartment are irregular, and the sectional area varies within wide limits. Many sacs 

 in this condition have been examined. Every compartment contains more than one 

 nucleus ; most can be seen to possess five or more. The upper part of the micropylar 

 end of the sac of fig. 12 is shown in fig. 11. The cytoplasm is irregularly vacuolated 

 and shows no sign of septation ; the nuclei are fairly uniform in size and appearance, 

 both among themselves and with the free nuclei enclosed in the lower compartments. 

 Their distribution in the cytoplasm is irregular. Area for area, the number of nuclei 



the upper unseptate part of the sac is smaller than in the lower half. Accord 



3 



to Lotsy ^, this micropylar end of the sac remains unseptate and the nuclei unchanged 

 until fertilization occurs. 



In the septate chalazal region of the sac the nuclei in each compartment become 

 massed together, and the constituents of each mass fuse to form a single nucleus. The 



o 



process of fusion, as in Welwitschia, appears to be of a simple character. The conti 

 portions of the nuclear membranes cease to be visible (PL 32. fig. 15), and gradually 

 the outline of the whole mass assumes a more regular form, in which the individuality 

 of the fusing nuclei is lost. 'For a time each fusion-nucleus contains several nucleoli 

 (fig. 16), which, however, are soon reduced to a single one, doubtless by fu 



(figs. 19, 22). The fusions are by no means simultaneous, for in a single sac will 



» Lotay, 1899, p. 92, fig. 28, f Coulter, 1908. J Pearson, 1 909, p. 347 



§ 





II Of. 



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Cf. 



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