46 COMPARATIVE EMBRYOLOGY. 



CHAPTER VI. 



COMPARATIVE EMBRYOLOGY. 



ONTOGENY 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 this ovmn, by rapid 

 alternation of growth (or increase in bulk) and asexual 

 reproduction (or binary fission), is 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 Larval 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 Echinodermata- and occurs in 

 Amphioxus. 



