FROM EGG TO INSECT SNODGRASS 387 



the deciding tiling, it ma}' be that the real determining influence 

 is something that escapes the scrutiny of the microscopist, and 

 that the chromosome combination is merely coincident with it. 

 This doubt is raised by the fact that in some plants and in some 

 of the lower animals, sex is not necessarily fixed at the time of 

 fertilization, and may either be established; later or influenced 

 by external circumstances. It is certain, however, that with the 

 great majority of animals of all kinds sex is determined at the 

 time of fertilization and that nothing can change it subsequently. 



Certain other characters that will appear in the new individual 

 as a part of its inheritance, besides those of sex, are known to be 

 linked with the chromosomes, but these are mostly traits that char- 

 acterize individual lines of descent, or varieties within the species. 

 The determining factors of the inheritance of fundamental species 

 characters by individuals of both sexes are supposed to be contained 

 in the bodies of the germ cells themselves. 



DEVELOPMENT 



Growth and development are not the same thing, but they are 

 so closely associated in animals that usually the animal continues 

 to develop as long as it grows, and grows only during its develop- 

 mental period. 



Growth takes place through a repeated division of the egg cell 

 or of a part of it to form a mass of cells that continue to multiply 

 by similar divisions. Development consists of a gradual, definite 

 arrangement in the growing mass of cells to form the various 

 parts and organs of the future adult — a new individual having 

 all the structural details characteristic of the species to which it 

 belongs, as well as some or all of the peculiarities of its parents 

 and immediate ancestors, but having also, in most cases, minor 

 characters of its own. Thus, heredity keeps each new individual 

 close to the ancestral path and maintains the line of descent; but 

 the freedom of each individual to deviate slightly from the ances- 

 tral path has made evolution possible, and has resulted in the vast 

 number of species of animals and plants in the world to-day. 



THE liEGINNING OF DEVELOPMENT 



With those species of animals whose eggs contain only a small 

 amount of yolk, development starts with the division of the entire 

 egg into two cells. A few insect eggs develop by entire division, but 

 most of them are so enlarged with 3'^olk, like a bird's egg, that 

 it is only the neucleus and a small amount of the surrounding 



