Principles of Animal Development. 



By John Beard, D.Sc, University Lecturer on Comparative Embryology 

 and Vertebrate Morphology, Edinburgh. 



I. A WheWs Egg. 



" Omne vivum ex ovo " is now an ancient aphorism, and it has become 

 a commonplace to say that the starting-point of almost all animal 

 development at the present time is the fertilised egg. The phenomena 

 leading up to this need not concern us at present. Let us assume 

 that we have got our fertilised egg, and let us in the first place con- 

 sider what sort of a thing it is. If the choice of such an egg be given, 

 how great is the variety out of which it may be selected. It may be 

 the microscopically small egg of an Echinoderm, or one of larger size 

 from the cocoon of an earthworm, the still bigger one from the leathery 

 egg-case of a whelk, or the huge one from the marvellous egg-capsule 

 of a smooth skate. It may be so large and contain so much food- 

 material that the developing organism may feed upon it for two years, 

 or so small and so destitute of nourishing substances that it can suffice 

 for the needs of the development for but a few hours. 



It may be destitute of any envelope beyond a thin structureless 

 membrane formed by the egg-cell itself, or it may be closely wrapped 

 up in a series of coverings, and outside of these we may find a shell 

 or case of complex form and architecture. 



The eggs of animals are, indeed, endless in variety, when the size, 

 amount of food-yolk, coverings and appendages, and the modes in 

 which they are deposited are taken account of. But with all this one 

 fact stands out quite clearly, that the size, composition, mode of de- 

 position, etc., of any particular egg, have an intimate relation and 

 connection with the development of an individual of the species whose 

 egg it is. 



The individual peculiarities of ova raise various interesting ques- 

 tions which have hitherto received but the barest consideration in 

 embryological text-books and even in developmental researches. Prob- 

 ably for solutions of most of them the science will have long to wait. 

 The nature of most modern embryological research is, unfortunately, 

 not such as to furnish hints in the direction of their elucidation ; 



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