DEVELOPMENT OF EGGS IN MANY-CELLED ANIMALS 335 



constantly rotating. Vegetative propagation is effected by bud- 

 ding, some of the individuals dividing repeatedly to constitute 

 daughter- colonies that are set free into the central cavity of 

 the parent colony, which ultimately breaks up and liberates 

 them. But development can also be effected in quite a different 

 manner, i.e. by the formation of eggs. Some of the members of 

 the colony increase in size and assume a spherical shape. These 

 are the eggs, or, to speak more accurately, egg-cells or ova. Other 

 members of the colony exhibit a similar increase, after which they 

 divide into a multitude of small orange-coloured cells, which are 

 set free into the surrounding water, and swim about actively by 

 means of the flagella with which they are provided. They are 

 known as sperms. Each ovum is fertilized by the permanent 

 fusion with it of a single sperm, a process which is curiously 

 like what happens during the conjugation of the Bell-Animalcule 

 (p. 325), where a small free-swimming bell is completely incor- 

 porated with a large fixed one. And here also the fusion imparts 

 increased vigour, which is manifested by active fission. But in 

 the Bell-Animalcule the products of division separate from one 

 another and become solitary adults, while in Volvox the corre- 

 sponding products remain united together, and collectively make 

 up a new colony. 



DEVELOPMENT OF EGGS IN MANY-CELLED ANIMALS 



(Metazoa) 



If vegetative propagation be left out of consideration, we may 

 say that every many-celled animal, from a Sponge to a Vertebrate, 

 begins its life-history as an egg-cell or ovum, which is fertilized, 

 in the vast majority of cases, by fusion with it of a sperm, derived, 

 as a rule, from another individual. The fertilized ovum then 

 actively divides to form a little mass of cells, which, by further 

 divisions and specializations, gradually shape themselves into the 

 tissues and organs of the adult. There is, in fact, a gradual 

 evolution of the individual {ontogeny), from a simple unit-mass of 

 living substance, and that this should be so is broadly interpreted 

 by the Law of Recapitulation as the result of inheritance, whereby 

 is repeated the evolutionary history ^phylogeny) of the particular 

 group of animals to which the developing individual belongs. 

 And if this interpretation be correct, the first stage in the life- 



