290 LEISURE-TIME STUDIES. 



use and habit, and through impressions or stimuli of special 

 kind having been made upon particular parts of the body. 

 There is little need to pursue this idea further, since the 

 theory of nerve-origin lies literally in a nutshell, and derives 

 its feasibility from the reasonableness of its assertions. 

 Given an animalcule with a sensitive body-substance ; admit 

 that its body becomes stable so as to present certain parts 

 to the outer world j and that, through use and want, impulses 

 come to travel in particular lines from these parts, and so to 

 produce changes and contractions in its internal structure 

 and we have outlined the essential details of the only scien- 

 tific and consistent theory which can account for the genesis 

 of muscle and nerve in living beings. 



The development of nerves in the animal world at large, 

 however, bears a very distinct relation to the development 

 of nerve-centres and sensory-organs in the highest of animals. 

 Can the development of the nervous system in higher animals 

 be said to throw any light upon the manner in which nerves 

 and sense-organs have originally arisen, namely, through the 

 contact of impulses with certain outward parts of a living 

 being, and through the subsequent relationship which be- 

 came established between these outward portions and the 

 inner structures of the organism ? We have already as- 

 signed to the study of development a paramount place, as 

 showing us the manner of origin of the organs and parts of 

 living beings. Let us inquire if the development of the 

 highest animals throws any light on the source and begin- 

 nings of their nerves ? 



The egg or germ of a vertebrate animal exists as a small, 

 or it may be microscopic, mass of protoplasm, known as the 

 "ovum," or "egg," exhibiting all the features of a "cell." Man 

 himself springs from such a body, which in his case attains a 

 diameter not exceeding the one hundred and twentieth part 

 of an inch. When the ovum exhibits the process of develop- 

 ment which results in the production of a new being, its 

 substance divides and subdivides (Fig. 53, A, B, c, D) in a 

 regular fashion into a mass of cells ; the egg being said, in 



