CHAPTER IX. 

 ONTOGENESIS. 



Every living thing begins its existence as a single cell, 

 a condition of primitive simplicity, and finally arrives at 

 a varying degree of complexity, according to its phy- 

 togeny. The study of the intervening transformations 

 through which each organism must pass is known as 

 ontogenesis or ontogeny. During the early stages of de- 

 velopment, there is no resemblance between the parent 

 organism and the growing germ which is known as an 

 embryo. The study of embryos and their development 

 is called embryology. 



When the embryo of one of the higher animals reaches 

 a certain point, and has developed sufficiently to enable 

 its specific characters to be recognized, it becomes 

 known as &fcetus. 



If, as in certain cases, the embryo becomes self-sustain- 

 ing and independent, without attaining parental re- 

 semblance and continues for some time in this "semi- 

 developed" form, it is described as a larva. In a few 

 forms of life, the larvae of the tapeworms Coenurus and 

 Echinococcus, and in certain dipterous midges, a peculiar 

 form of parthenogenetic reproduction takes place in the 

 larvae. To it the term pozdogenesis has been applied. 



The early writers upon the science of development, 

 having no data upon which to build, were obliged to 

 content themselves with theoretical speculations, most 

 of which are of little interest to-day, yet they deserve 

 consideration and are useful as exemplifying how wide 

 of the truth theory may lead and how difficult it may 

 be for it to give place to fact. 



Until the time of William Harvey (1578-1657) the 

 whole subject of "generation" was so obscure as to 



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