88 THE BIOLOGY OF AN ANIMAL. 



tiallj true of all animals above the earthworm, as well as of 

 many below it — of all, in a word, in which the three germ- 

 layers are developed, i.e., all those above the Cmleiiterata^ or 

 polyps, jelly-fishes, hydroids, sponges, etc. In man, as in the 

 earthworm and all intermediate forms, the ectoblast gives rise 

 to the outer skin (epidermis), the brain and nerves, fore- and 

 hind-gut ; the entoblast gives rise to the lining membrane of the 

 stomach, intestines, and other parts pertaining to the mid-gut; 

 while the somatic and splanchnic layers of the mesoblast give 

 rise to the muscles, kidneys, reproductive organs, heart, blood- 

 vessels, etc. It is now generally held that the germ-layers 

 throughout the animal kingdom (with the partial exception of 

 the Coelenterata already mentioned) are essentially identical in 

 origin and fate. This view is known as the Germ-layer Theory. 

 It is one of the most significant and important generalizations 

 which the study of Embryology has brought to light, since it 

 recognizes a structural identity of the most fundamental kind 

 among all the higher animals. 



Sooner or later the young earthworm bursts through the 

 walls of the capsule and makes its entry into the world. When 

 first hatched it is about an inch long and has no clitellum. 



It is a curious fact that in certain species of Lumbricus the young 

 worms are almost always hatched as twins, two individuals being derived 

 from a single egg by a process which is described by Kleinenberg in the 

 Quarterly Journal of Microscopical Science, Vol, XIX., 1879. It often 

 happens that the twins are permanently united by a band of tissue, as in 

 the case of the well-known Siamese twins. 



We have now traced roughly the evolution of a complex 

 many-celled animal from a suuple one-celled germ. It is im- 

 portant to notice at this point a few general principles which are 

 true of higher animals in general. 



1. The embryological history is a true process of develop- 

 ment, — not a mere growth or unfolding of a pre-existing rudi- 

 ment as the leaf is unfolded from the bud. ISTeither the ovum 

 nor any of the earlier stages of development bears the slightest 

 resemblance to an earthworm. The embryo undergoes a trans- 

 formation of structure as well as an increase of size. 



2. It is a progress from a one-celled to a many-celled con- 

 dition. 



