The Development of the Animal World 69 



the leading authorities that this remarkable reminiscence 

 of earlier ancestors really occurs in an animal's em- 

 bryonic development, and we shall see some very curious 

 and beautiful illustrations of it as we proceed. For the 

 earlier stages we have only to watch the embryonic 

 development of some low type of animal — a coral, or a 

 sponge, or even a worm — and we find the series of forms 

 as I have just suggested it. Each animal is at first a 

 tiny single cell, and in its immature stage this cell — the 

 ovum or egg — is amoeboid. It divides and sub-divides until 

 it forms a round cluster with a hollow centre. The 

 round hollow ball doubles in on itself (or "invaginates"), 

 and — in free-swimming embryos at this stage — the outer 

 layer of cells attends to locomotion, the inner layer to 

 digestion. 



On these lines we conjecture with some confidence 

 what the early stages of animal evolution were. The 

 single cell is capable of a vast amount of development 

 while remaining a single cell, as the populous world of 

 the Protozoa — one-celled, microscopic animals— shows. 

 Some remain of the Amoeba type, with a rough kind of 

 temporary limb and no permanent mouth. Others 

 develop permanent organs of locomotion, lashes (cilia or 

 flagella), with which they beat the water like oars, and a 

 permanent mouth and gullet. Others attach themselves 

 to long fixed stalks — like the Vorticella — and have a 

 crown of cilia round the mouth by which they make little 

 whirlpools in the water and bring the food to them. 

 Others shoot out their plasm in long star-like streamers 

 — like the pretty Heliozoa — and catch their food in it ; 

 some develop these into a kind of dart or harpoon. 

 Others grow a hard and often beautiful shell, and we get 

 the Foraminifers and Radiolaria; while others take to 

 parasitism, and may develop piercing and suctorial 

 organs. 



