300 THE EMBRYO [CH. 



as well as the origination of organs. It was natural to trace this method 

 back to the embryo, especially in cases where the cleavages are as clearly 

 marked as they are in the Leptosporangiatae, the Ferns which naturally 

 were examined first. Thus for a time the study of cell-cleavages dominated 

 genetic morphology, finding a favourable centre in the embryology of such 

 Ferns as had happened to be the best known. 



In Chapter XIV of the Land Flora this position has been critically 

 examined, and the conclusion stated (p. 180) that instead of accepting a 

 general embryology as based upon cell-cleavages, it would be more natural to 

 regard the embryo as a living whole: to hold that it is liable to be segmented 

 according to certain rules at present little understood : that its parts are 

 initiated according to principles also as yet only dimly grasped : and that 

 there may be, and sometimes is, coincidence between the cleavages and the 

 origin of the parts, but that the two processes do not stand in any obligatory 

 relation one to the other. In particular this conclusion was found to apply 

 to the embryonic organ called the "foot." The inconstancy of its position, 

 and even of its existence in various types, suggests that it is an organ 

 formed only where it is required for the first stages of development and 

 nutrition of the embryo. This idea has found support in the embryological 

 facts disclosed in the Lycopods and Ophioglossaceae. Later von Goebel 

 formulated a general opportunist position as applied to the embryo: viz. that 

 root, shoot, and haustorium are laid down in the positions that are most 

 beneficial for their function: that external forces do not come into con- 

 sideration in the arrangement in space of the parts of the embryo ; and 

 accordingly that we have only to consider internal factors. (Von Goebel, 

 Organographie, 1898, p. 452, English Edn. p. 246.) 



A revision of the embryology of the whole series of Pteridophytes leads, 

 however, to the conclusion that the form of the embryo is not so plastic as 

 this would imply (see Land Flora, Chapter XLII). Comparison shows that 

 the polarity of the embryo is indicated by the first segmentation of the 

 zygote. Of this segmentation there are two types according as a suspensor 

 is present or absent: otherwise it shows remarkable constancy. Where a 

 suspensor is formed in Ferns, the first segment-wall divides the zygote 

 approximately at right angles to the axis of the archegonium : the cell 

 nearer to the neck forms the suspensor, the other is the embryonic cell. 

 Where there is no suspensor the zygote is itself the embryonic cell, and the 

 first cleavage is not necessarily transverse to the axis of the archegonium. 

 In either case the embryonic cell is ultimately divided into octants by 

 cleavages at right angles to one another (Fig. 284). The two types, with and 

 without a suspensor, are represented both in the Lycopods and in the Fili- 

 cales. In all fully investigated cases the octant-division takes place, though 

 the sequence of the cleavages is not uniform : and the embryo is accordingly 



