I 24 Darwin, and after Darwin. 



all round the circumference of the pellucid envelope, 

 as represented in this illustration (Fig. 32). 



In thus saying that the ova of all animals are, so 

 far as microscopes can reveal, substantially similar, I 

 am of course speaking of the egg-cell proper, and 

 not of what is popularly known as the &%Z- The q^^ 

 of a bird, for example, is the egg-cell, plus an enor- 

 mous aggregation of nutritive material, an egg-shell, 

 and sundry other structures suited to the subsequent 

 development of the egg-cell when separated from the 

 parent's body. But all these accessories are, from 

 our present point of view, accidental or adventitious. 

 What we have now to understand by the ovum, the 

 ^gg' or the egg-cell, is the microscopical germ which I 

 have just described. So far then as this germ, is 

 concerned, we find that all multicellular organisms 

 begin their existence in the same kind of structure, 

 and that this structure is anatomically indistinguishable 

 from that of the permanent form presented by the 

 lowest, or unicellular organisms. But although anato- 

 mically indistinguishable, physiologically they present 

 the sundry peculiarities already mentioned. 



Now I have endeavoured to show that none of 

 these peculiarities are such as to exclude — or even so 

 much as to invalidate - the supposition of develop- 

 mental continuity between the lowest egg- cells and 

 the highest protozoal cells. It remains to show in this 

 place, and on the other hand, that there is no breach 

 of continuity between the lowest and the highest egg- 

 cells ; but, on the contrary, that the remarkable 

 uniformity of the complex processes whereby their 

 peculiar characters are exhibited to the histologist, is 

 such as of itself to sustain the doctrine of continuity 



