50 Animal Life and Intelligence. 



cells is developed a heterogeneous system of different but 

 mutually related tissues. 



This view of development is, however, the outcome of 

 comparatively modern investigation and perfected micro- 

 scopical appliances. The older view was that development 

 in all cases is nothing more than differential growth, that 

 there is no differentiation of primitively similar into 

 ultimately different parts. Within the fertilized ovum of 

 the horse or bird lay, it was supposed, in all perfection of 

 structure, a miniature racer or chick, the parts all there, 

 but too minute to be visible. All that was required was 

 that each part should grow in due proportion. Those who 

 held this view, however, divided into two schools. The 

 one believed that the miniature organism was contained 

 within the ovum, the function of the sperm being merely 

 to stimulate its subsequent developmental growth. The 

 other held that the sperm was the miniature organism, the 

 ovum merely affording the food-material necessary for its 

 developmental growth. In either case, this unfolding of 

 the invisible organic bud was the evolution of the older 

 writers on organic life. More than this. As Messrs. 

 Geddes and Thomson remind us,* "the germ was more 

 than a marvellous bud-like miniature of the adult. It 

 necessarily included, in its turn, the next generation, and 

 this the next — in short, all future generations. Germ 

 within germ, in ever smaller miniature, after the fashion 

 of an infinite juggler's box, was the corollary logically 

 appended to this theory of preformation and unfolding." 



Modern embryology has completely negatived any such 

 view as that of preformation, and as completely established 

 that the evolution is not the unfolding of a miniature germ, 

 but the growth and differentiation of primitively similar 

 cell-elements. In different animals, as might be expected, 

 the manner and course of development are different. We 

 may here illustrate it by a very generalized and so to 

 speak diagrammatic description of the development of a 

 primitive vertebrate. 



* " The Evolution of Sex," p. 84. 



