6 Heredity and Environment 



I. Development of the Body 



The entire individual — structure and functions, body and mind 

 — develops as a single indivisible unity, but for the sake of clarity 

 it is desirable to deal with one aspect of the individual at a time. 

 For this reason we shall consider first the development of the 

 body, and then the development of the mind. 



i. The Germ Cells. — In practically all animals and plants in- 

 dividual development begins with the fertilization of a female sex 

 cell, or egg, by a male sex cell, or spermatozoon. The epigram 

 of Harvey, "Omne vivum ex ovo," has found abundant confirma- 

 tion in all later studies. Both egg and spermatozoon are alive 

 and manifest all the general properties of living things. How 

 little this fact is appreciated by the public is shown by the repeated 

 announcements of the newspapers. that someone who has made an 

 egg develop without fertilization "has created life." An egg or a 

 spermatozoon is as much alive as is any other cell ; as character- 

 istically alive as is the adult animal into which it develops. 



What is Life? — It is difficult to define life, as it is also to define 

 matter, energy, electricity, or any other fundamental phenomenon, 

 but it is possible to describe in general terms what living things 

 are and what they do. Every living thing whatever, from the 

 smallest and simplest micro-organism to the largest and most 

 complex animal, from the microscopic egg or spermatozoon to the 

 adult man, manifests the following distinctive properties: 



(a) Protoplasmic and Cellular Organization. — It contains pro- 

 toplasm, "the material basis of life," which is composed of the 

 most complex substances known to chemistry. Protoplasm is not 

 a homogeneous substance, but it always exists in the form of 

 cells, which are minute masses of protoplasm composed of many 

 distinct parts, the most important of these being the nucleus and 

 the cell-body (Fig. i.) 



The nucleus is a central rounded body usually denser than the 

 surrounding cytoplasm from which it is separated by a thin mem- 

 brane. It contains granules or threads of a substance which has 



