144 THE EVOLUTION OF MAN. 



are quite visible to the naked eye ; but the greater number 

 are microscopic. Our reasons for regarding the Amoebae as 

 the particular one-celled organisms, the phylogenetic rela- 

 tions of which to the egg-cell are of peculiar importance, 

 will be evident from the following facts. In many lower 

 animals, the egg-cell remains in its original, naked condition 

 till it is fertilized ; it acquires no covering, and is often 

 indistinguishable from an Amoeba. Like the latter, these 

 naked egg- cells can extend processes and move about. In 

 the Sponges, these active egg-cells creep freely about, as 

 though they were independent Amoebse (Fig. 14), even 



/ I Fig. 14. — Egg-cell of a Chalk Sponge (Olyn- 



pO '^i ) thus). The egg-cell moves and creeps about within 



fX L!i^|^\ yO^ ^^® Sponge, by means of variable processes which 



C\^l JiS^--^'fSr it extends. It is not distinguishable from the 



k^s«^^^X%a common Amoeba. 



( '^^ /f^V^^'r^ within the parent orojanism. In this 

 /^^^ IrS) condition they were observed by earlier 



naturalists, and were mistaken for 

 Amoebae, living as parasitical intruders in the body of 

 the Sponge. It was only afterwards that it was dis- 

 covered that these supposed one-celled parasites were in 

 reality the egg-cells of the Sponge itself This remarkable 

 phenomenon is also found in other lower animals, for ex- 

 ample, in those pretty bell-shaped Plant-animals (Medusce) ; 

 the eggs of these also remain as naked, uncovered 

 cells, which stretch out amoeboid processes, feed themselves, 

 move, and from which, after fertilization, the many-celled 

 Medusa-organism is indirectly or directly developed by 

 repeated division. 



It is, therefore, certainly no wild hypothesis, but an 



