CHAPTER XV 



THE EMBRYO 



The act of Syngamy, consisting in the fusion of the spermatozoid with the 

 ovum, produces the zygote, which is the starting-point for the diploid gene- 

 ration or sporophyte. So far as is known for Ferns at large the process is 

 uniform, and the characters of the gametes involved in it are so similar that 

 no comparative arguments have hitherto been based upon them. It is true 

 that the spermatozoid of Marsilia is a body with many coils of its spiral 

 form, while that in most Ferns has fewer coils. But at present the detailed 

 facts are deficient for the Filicales as a whole, and no satisfactory use can be 

 made of them till a better position is reached. The prevailing uniformity of 

 the ovum is equally a barren source for comparative data. At present it is 

 not from microscopic observation of the gametes themselves that assistance 

 in phyletic argument can come. The zygote which results from their fusion 

 is at first a primordial cell, which at once secretes a pellicle of cell-wall ; and 

 the roughly spherical cell thus produced, sunk in the venter of the arche- 

 gonium, presents a very uniform starting-point for Fern embryology. It has 

 been seen how uniformly standardised is the archegonium in Ferns. It seems 

 a natural consequence that at first the zygote which it contains should also 

 appear thus standardised. But the surroundings of the zygote as it develops 

 into the embryo are by no means uniform in Ferns at large. The form of 

 the gametophyte which bears the archegonium, its relation to external con- 

 ditions of aeration and of light, the position of the sources of food relative 

 to the archegonium, and in particular the orientation of the archegonium 

 itself, are so diverse in the different types, that it might well be expected 

 that divergent details should appear in embryos so differently placed. 



In the embryo, as is the case of any shoot or part of the adult plant, two 

 factors influencing development must be considered. First the qualities in- 

 herited from the ancestry, and secondly the features resulting from the 

 impress of external circumstance. The theoretical problem in all embryology 

 of plants will be to realise in the examination of the embryo as it is, how 

 the balance has been struck between the first and the second of these factors, 

 which, working together, may be held to have produced the effect which we 

 see. In earlier days the tendency was to lay the greater stress upon the 

 inherited factor ; and indeed to translate the observed details directly into 

 terms of Descent. More recently a tendency has arisen to refer the form 

 of the embryo largely to the biological conditions under which it develops. 



