ORGANIC EVOLUTION 175 



inverted flask-shaped form, and are then known as chlora- 

 gogue cells. 



These are but the barest outlines of the principal develop- 

 mental processes. We have not time to enter farther into 

 the field of embryology. We have gone far enough to see 

 that the development of an organism from an egg is a truly 

 wonderful process. We need but go back again and look at 

 the marvelous simplicity of the egg to be convinced of it. 

 Not only do cells differentiate, but cell groups act together 

 like well drilled battalions, cleaving apart here, fusing 

 together there, forming protective coverings or communicat- 

 ing channels, apparently creating out of nothing, a whole 

 set of nutritive and reproductive organs, all in orderly and 

 progressive sequence, producing in the end that orderly dis- 

 posed cell aggregate, that individual life unit, which we 

 know as an earthworm. Although the forces involved are 

 beyond our ken, the grosser processes are evident, and may 

 be summarized as follows : 



1. In respect to development, the general phenomena 

 are : cell multiplication and cell differentiation. 



2. The principal changes of form and relations are: 

 segmentation, gastrulation, formation of the mesoderm, 

 splitting of the mesoderm, formation of the anus, origin of 

 the nervous system from the ectoderm, origin of the nephri- 

 dia and reproductive organs from the mesoderm, ingrowth 

 of the ectoderm to form stomodaeum and proctodeum. 

 These are the A, B, C's of embryology. 



3. The derivation of the principal tissues from the three 

 germ layers is: From the ectoderm: 



epidermis and setae 



lining of stomodaeum and proctodeum 



the whole nervous system. 

 From the endoderm: the alimentary epithelium 

 From the mesoderm : 



