FACTORS IN EMBRYOGENESIS 25 



enlargement, as do the adjacent cells of the foot; (ii) the presence of a 

 suspensor indicates that there are very different physiological conditions 

 at the neck as compared with the basal end of the archegonium. We 

 may also note that a suspensor is consistently present in the embryogeny 

 of seed plants. Whether the very different developments of the sus- 

 pensor and the embryonic cell in pteridophytes are due to a gene- 

 controlled differentiation in the egg or zygote, to metabolic gradients 

 in the prothallus, to gradients of gaseous concentration, or to some 

 residual effect of fertilisation, must remain open questions. Certainly, 

 there would appear to be some quite fundamental difference between 

 those pteridophytes in which the epibasal cell (destined to become the 

 shoot apex) lies adjacent to the neck of the archegonium, as in Psilotum, 

 and those in which it is adjacent to the base of the archegonium, as in 

 Lycopodium} 



When the comparative morphologist asserts that the suspensor is a 

 primitive organ found in the embryogeny of some modern survivors of 

 ancient stocks, he has probably seized upon a truth, even though he 

 cannot account for the mechanism involved : it is a particular feature 

 of the hereditary constitution which happens to become manifest in the 

 embryogeny. In those genera and families in which a suspensor is 

 known, it occurs with a high degree of regularity. In the contemporary 

 view, the suspensor could be regarded as a gene-determined organ in 

 the inception of which, perhaps, only a few genes are involved ; and 

 the genie action leading to its formation would be seen in the differential 

 protoplasmic changes which take place in the elongating zygote. Even 

 if we accept D'Arcy Thompson's view that physical factors determine 

 the way in which the zygote divides, the material on which the 

 relevant forces must work is a specific protoplasm, the metabolism 

 of both the zygote and the surrounding cell matrix being under genie 

 control. 



In this, as in all studies of morphogenesis, the basic assumption is 

 that genie action pervades every phase of development, i.e. it is involved 

 in the metabolism which underlies all morphological and histological 

 developments. The distinctive biochemical situations which arise at 

 different stages in the ontogeny have also their biophysical expression. 

 Indeed, at the molecular level, the distinction between chemical and 

 physical properties largely disappears. The action of various extrinsic 

 factors may tend to obscure the primary genie effect, but, as has long 

 been recognised, the basic material on which all morphogenetic factors 

 work is the specific hereditary substance, the ultimate form and structure 

 being the result of both internal and external factors. 



This brief survey shows how very little exact information there is on 



1 See p. 103, Fig. 5. 



