46 COMPARATIVE EMBRYOLOGY. 
CHAPTER VI. 
COMPARATIVE EMBRYOLOGY. 
NTOGENY is the development or production of the 
individual. The study of ontogeny is Embryology. 
The individual dates its existence from the fertilised 
ovum sexually produced from the male and female cell- 
elements. From this and other considerations it will be 
seen that there is no true ontogeny in the Protozoa or 
unicellular animals, for they are produced asexually (by 
binary or multiple fission) from their parents. 
The first important fact about the ontogeny of the 
Metazoa is that they commence life as a single cell, the fertilised 
ovum. The second point to notice is that th7zs ovum, by rapid 
alternation of growth (or increase in bulk) and asexual 
reproduction (or binary fission), zs transformed into the 
multicellular adult. The third is the differentiation of the 
multicellular organism from a homogeneous cell-mass to a 
heterogeneous structure, in accordance with the law of physi- 
ological division of labour. This process, in the vast majority 
of instances, takes place step for step along with the increase 
in cells, because the individual requires to be a working 
organism at every stage of its development. At any develop- 
mental stage the organism, as when adult, has a definite 
environment to which its structure and vital activities must 
correspond or it would perish. 
Larva and Embryo.—The environment of developing 
organisms shows an infinite variety, but for purposes of 
convenience we may distinguish at least two extremes. In 
the first, called the Zarva/ type, the ovum, either at the 
very outset or before development has proceeded far, is 
freed from the parent and lives and fights for itself in the 
outside world until, after many changes, it becomes an adult. 
This type is common in L£chinodermata and occurs in 
Amphioxus. 
