FACTS AND FACTORS OF DEVELOPMENT 523 



climax of all wonders — greater even than that involved in the evolution 

 of a species or in the making of a world. 



The fact of development is everywhere apparent; its principal steps 

 or stages are known for thousands of animals and plants ; even the pre- 

 cise manner of development and its factors or causes are being success- 

 fully explored. Let us briefly review some of the principal events in 

 the development of animals, and particularly of man, and then consider 

 some of the chief factors and processes of development. Most of our 

 knowledge in this field is based upon a study of the development of 

 animals below man, but enough is now known of human development to 

 show that in all essential respects it resembles that of other animals, 

 and that the problems of heredity and differentiation are fundamentally 

 the same in man as in other animals. 



I. Development of the Body 



The entire individual — structures 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. 



1. The Germ Cells. — In practically all animals and plants individual 

 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 confirmation in all later studies. 

 Both egg and spermatozoon are alive and manifest all the general prop- 

 erties of living things. How little this fact is appreciated by the public 

 is shown by the repeated announcements by the newspapers that " Pro- 

 fessor So-and-so has created life because he has made an egg develop 

 without fertilization." An egg or a spermatozoon is as much alive as is 

 any other cell — as characteristically alive as is the adult animal into 

 which it develops. 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 microorganism to the largest and most complex animal, from 

 the microscopic egg or spermatozoon to the adult man, manifests the 

 following distinctive properties : — 



1. It contains protoplasm, "the material basis of life," which is com- 

 posed 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 cytoplasm 

 (Fig. 1). Protoplasm is therefore organized, that is, composed of many 

 parts all of which are integrated into a single system, the cell. Higher 

 animals and plants are composed of multitudes of cells, differing more 



