768 LECTURE XLTII. 



capable of development by taking up the fertilising substance which is brought to 

 it by another cell (or another gamete) generally a zoosperm (antherozoid), or in the 

 case of the phanerogams by a pollen-tube. Now it would be somewhat absurd to 

 suppose that this fertilising substance is of exactly the same constitution as that of 

 the oosphere itself; though something of the sort might perhaps be supposed in such 

 cases as those of the Conjugateae, IMucorini and some Gamosporeae, where the two 

 conjugating cells appear to be externally of like constitution. 



On this assumption fertilisation would merely consist in a union of the substance 

 of the reproductive cells. That this is just not the point, however, is shown by all 

 those cases where a relatively large oosphere is fertilised by a very minute zoosperm, 

 the whole substance of which scarcely amounts to the thousandth part of the mass 

 of the oosphere, and the same conclusion follows naturally from all observations 

 on the behaviour of the pollen-tube when it fertilises the oosphere of a seed- 

 forming plant. Even the different shapes of the two sexual cells — of an antherozoid 

 or a pollen-grain compared with the oosphere — indicate definitely that both are 

 constituted differently as to material, since the external form as well as the internal 

 structure of any body are the necessary expression of its material constitution. 

 Difference of form always indicates difference of material substance. 



We can thus say, then, the fertilisation of an oosphere (or of a gamete) 

 consists in that something is added to its substance which was hithe?-to watiting to 

 it, but which it needs for further development. What this fertilising substance may 

 be is still in a high degree doubtful ; at any rate it is not the whole mass of an 

 antherozoid, and much less the whole mass of a pollen-grain which can answer 

 to the title of fertilising substance. The extremely small quantity of the latter, 

 however, generally effects conspicuous alterations in the oosphere at once : the 

 excretion of a cell-wall, growth and cell-division, the formation of the embryo, and 

 finally all those successive changes which take place in the neighbouring parts of 

 the mother-plant itself — the completion of the seed and walls of the fruit — and not 

 rarely the consequences of fertilisation extend over the whole plant, which at length 

 dies down after the ripening of the seeds. 



If in the case of so extremely peculiar a natural process as fertilisation it is 

 permissible to look for an analogy at all, we might be reminded especially of the 

 action of the ferments, which at least present a similarity in so far that ex- 

 tremely small quantities of them are able to put in motion large masses of 

 matter, and thereby chemically alter the latter. There is not much gained 

 by this however, since the action of ferments is itself as yet not understood. 

 Fertilisation also presents similarity with many phenomena of irritability, though that 

 again does not say much for our point of view, since we may say that fertilisation is 

 the most pronounced of all phenomena of irritability : I understand irritability to be, 

 as already stated, the mode of action of a living organism towards all external 

 stimuli, and therefore fertilisation belongs obviously to this category, the oosphere 

 being the organism which, under the stimulus from without of the fertilising 

 substance coming into contact with it, reacts in so astonishing a manner that new 

 processes of configuration and growth arise from it. 



In asexually produced spores, however, we find cells capable of repro- 

 duction which, without the addition of a fertilising substance nevertheless 



