CHAPTER XXIII* 



DEVELOPMENT 



Development is the evolution of a germ into a com- 

 plete organism. The study of the changes in the 

 developing embryo constitutes the science of Embry- 

 ology ; the transformations after the egg life are called 

 metamorphoses, and include growth and repair. 



The process of development is a passage from the 

 general to the special, from the simple to the complex, 

 from the homogeneous to the heterogeneous, by a series 

 of differentiations. It brings out 

 first the profounder distinctions, 

 and afterward those more external. a * t> mo c 



, , . i FIG. 361. Fertilization and 



That IS, the mOSt essential partS segmentation of mammalian 



appear first. And not Only does ovum: , spermatozoon; 



J n, nucleus ; nu, nucleo- 



development tend to make the sev- lus ; *, z n* nuiiata ; c i t 



. . segmenting cell. 



eral organs of an individual more 

 distinct from one another, but also the individual itself 

 more distinguished from other individuals and from the 

 medium in which it lives. With advancing develop- 

 ment, the animal, as a rule, acquires a more specific, 

 definite form, and increases in weight and locomotive 

 power. Life is a tendency to individuality. 



The first step in development, after fertilization, is the 

 segmentation of the egg, by a process of self-division. 

 In the simplest form, the whole yolk divides into two 

 parts ; these again divide repeatedly, making four, eight, 

 sixteen, etc., parts, until the whole yolk is subdivided 



* See Appendix. 

 409 



