NATURE OF EHBRTOLOGICAL DEVELOPMENT. 153 



and all intermediate forms, the cctoblast 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, intes- 

 tines, and other parts pertaining to the mid-gut ; while the somatic 

 and splanchnic layers of the mesoblast give rise to the muscles, kid- 

 neys, reproductive organs, heart, blood-vessels, etc. It is now gen- 

 erally held that the germ-layers throughout the animal kingdom 

 (with the partial exception of the ( '<> 1< n1< /'<tf<( 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 clitelluui. 



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

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

 from a single egg by a process which is described by Klei'.ienberg in the 

 Quarter?;/ Journal of Micro^-ojtn-ul Ki-ieitce, 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 simple 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 development, 

 not a mere growth or unfolding of a pre-existing rudiment, 



as the leaf is unfolded from the bud. Neither the ovum nor any 

 of the earlier stages of development bears the slightest resem- 

 blance to an earthworm. The embryo must undergo a trans- 



v o 



formation of structure as well as an increase of size. 



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

 tion. 



3. It is a progress from relative simplicity to relative com- 

 plexity. The ovum is certainly vastly more complex than it ap- 

 pears to the eye, but no one can doubt that the full grown worm 

 is more complex still. 



4. It is a progress from a slightly differentiated to a highly 



