FACTORS IN EMBRYOGENESIS 7 



gametophyte tissue exercises its principal effects both on the maturation 

 of the egg and the subsequent growth of the embryo. Before and after 

 fertihsation, the archegonium is a locus of growth and hence there is a 

 translocation of nutrients of all kinds to it from the surrounding tissue. 

 We may therefore assume, as a working hypothesis, that both the ovum 

 and the zygote are liable to be affected by metabolic gradients, and that 

 these may be among the factors which determine polarity and other 

 important developments. In different classes of Embryophyta, then, the 

 environment of the ovum and zygote deserve careful study. 



In the germination of a spore, be it of an alga, a bryophyte, or a 

 pteridophyte, the environment of the single propagative unicell is (i) 

 the water, soil, rock, bark or other substratum in or on which the spore 

 has been deposited, and (ii), in the case of subaerial organisms, moist 

 air. In all of these spores, there is no adjacent tissue either to supply 

 nutrients to, or to direct or control the morphogenetic developments 

 in, the germinating cell. The developments on germination are, there- 

 fore, determined, or directed, by internal factors, including the cyto- 

 plasmic organisation which has already been established, and by 

 factors in the external medium or environment, e.g. light, humidity, 

 solutes in the substratum, gravity etc. Comparisons of the growth and 

 development of naked as compared with enclosed germ cells, and an 

 analysis of the factors at work in them, in fact, yield many points of 

 interest, including some notable homologies of organisation as well as 

 evident dissimilarities, 



STATEMENT OF THE PROBLEMS — INITIAL DEVELOPMENT 

 IN DIFFERENT CLASSES 



The fertihsed egg is usually a spherical or ellipsoidal body and so 

 too are many spores, though spores may exemplify a considerable 

 diversity of shape. In Fig. 1 young embryos from different taxonomic 

 groups have been selected to illustrate the fact that certain develop- 

 ments are of very general occurrence and to provide materials for a 

 consideration of the main problems in embryogenesis. 



On spore germination, or at the earliest manifestation of growth in 

 the zygote, there is evidence of polarity and of filamentous or axial 

 development; in all but the very simplest organisms, there is, from the 

 outset, a distinction of apex and base (or of distal and proximal regions, 

 or poles). In short, both the sporeling and the very young embryo 

 afford evidence of orderly or organised development. Each species has 

 its own particular ontogenetic pattern from which there are, as a rule, 

 no very marked departures. If, at the outset of development, there is 

 an indeterminate phase, during which the morphogenetic process might 

 be directed into one of several possible channels or trends, that phase 



