i So EMBRYOLOGY 



may participate in the growth ; while, conversely, the growth may actually 

 extend in some cases to other octants than those cited. Further, in certain 

 cases, and especially in the Lycopodiales, the relative position of the parts 

 of the embryo is not constant. There are thus difficulties in the way of 

 according any high importance to the primary segmentations of the embryo. 

 From the facts as now known, it would appear 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 : 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 

 the one to the other. 



While the embryology based on cell-cleavages was developing, Sachs was 

 engaged in maturing his comprehensive views on the arrangement of cell- 

 walls in the youngest parts of plants. His recognition of the prevalence 

 of rectangular division of the cells, coupled with the demonstration that 

 the same mode of segmentation may occur in such diverse bodies as embryos 

 and superficial hairs, went far towards reducing the arrangement of cell 

 walls to a general rule : it became apparent that the first cleavages of the 

 embryo are not so much the indications of a phylogenetic history, as the 

 necessary consequences of rectangular division in a body of approximately 

 spherical form. And now that finally the demonstration has come that in 

 the continued embryology at the apex of stem and root the segmentation 

 has no strict or constant relation either to the formation of the appendages, 

 or to the internal differentiation of tissues in plants at large, the logical 

 foundation has been swept away from below the feet of the adherents 

 of arguments from cleavage. For here as elsewhere we are bound now 

 to admit that there is no necessary or constant relation between cell- 

 cleavages and differentiation, external or internal. Such relations may 

 exist, it is true, and they sometimes do but their inconstancy shows that 

 they cannot be made the subject of general argument. 



It will thus appear that the methods of embryology hitherto employed 

 require considerable revision, so as to bring them into line with the facts 

 already observed. Excepting perhaps within narrow circles of affinity, and 

 especially in those where definiteness is the rule, arguments from detail 

 of segmentation must be discounted : and this will be so in regard to the 

 initial embryogeny of the sporophyte, as much as to the continued embry- 

 ology close to the growing apex. Concurrently with the gradual acquisition 

 of the facts which have led to this general conclusion, there has grown up 

 a definite tendency of thought towards a new view of embryological facts. 

 The assumption of some unity of plan, or type of construction of the 

 embryo in Archegoniate plants, which so long dominated these comparisons, 

 has relaxed its hold : in its place has come the desire to study the young 

 sporophyte biologically, as a germ to be nursed by the parent plant, in the 

 Bryophytes till full maturity, but in the Vascular Plants for a time only, 



