Chapter III 

 FACTORS IN EMBRYOGENESIS (continued) 



THE FIRST DIVISION OF THE ZYGOTE 



THE fertilised ovum is usually seen and depicted as a spherical or 

 ellipsoidal body with no evident regional protoplasmic differentia- 

 tion. In many archegoniate plants it elongates in the axis of the 

 archegonium and soon reaches a size when cell division becomes 

 necessary. A partition wall is then laid down, the position of this wall 

 being generally, though not invariably, an indicator of the polarity, i.e. 

 the axis of the embryo is at right angles to it. The formation of this 

 first wall is interesting on several counts. According to D'Arcy 

 Thompson (1917, 1942), division is a means of restoring equilibrium 

 within an enlarging cell, and hence, in a considerable measure, the 

 histological pattern can be directly attributed to the action of physical 

 factors, i.e. to internal and surface forces. If an elongating ellipsoidal 

 zygote were of uniform protoplasmic consistency and metabolic 

 activity, with uniform conditions on all sides of it, it would, at a certain 

 critical size, divide into two exactly equal portions by a median trans- 

 verse wall, i.e. a wall of minimal area. The first wall would thus be at 

 right-angles to the embryo axis. Fig. 5. This is what takes place in many 

 bryophytes and in Psilotum, Tmesipteris (Holloway, 1917, 1921, 1939), 

 Equisetum and some other pteridophytes. In Anthoceros, however, the 

 ovoid zygote is divided by a longitudinal wall. Figs. 5, 14. In lepto- 

 sporangiate ferns, the spherical zygote is divided into two equal hemi- 

 spheres, though Atkinson (1894) and others have suggested that the 

 segments may be somewhat unequal. There are, however, many 

 pteridophyte embryos, notably those in which a suspensor {see below) 

 is formed, and also flowering plant embryos, where the first wall does 

 not divide the zygote into cells of equal volume, Fig. 5. If, therefore, 

 cell division is a means of maintaining an approximate state of equili- 

 brium in a growing system, it may be inferred that the enlarging zygote, 

 in certain species, must undergo some protoplasmic differentiation; 

 for the first wall separates a relatively large suspensor cell from the 

 relatively small embryo cell,^ Fig. 5. In other words, this unequal 



^ In pteridophytes in which no suspensor is present, the first partition wall divides the zygote 

 into an epibasa! ce\l, which gives rise to the shoot, and a hypobasal cqW, which gives rise to the root 

 and foot. But in Lycopodiiim, Selaginella, and some eusporangiate ferns, the first wall divides the 

 zygote into a suspensor and an embryonic cell proper. The latter then divides by a wall parallel 

 to the first wall, the cell next the suspensor being recognised as the hypobasal cell and the 



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