46 The Unity of the Organism 



concentric. From these layers all the organs and tissues 

 are developed by a great variety of unequal thickenings and 

 foldings and concentrations and cellular differentiations. 

 Details are not necessary for our purpose. As expressed by 

 one standard textbook of embryology, these layers are as 

 a rule "structural units of a higher order than the cells." 

 "Primary organs of the animal body" is anotlier term ap- 

 plied to them. The appropriateness of the descriptive term 

 "germinal" applied to these layers is found in the fact that 

 the tissues and organs are generated from them. 



The passage of the embryos of so many different animals 

 through this layered condition makes the phenomenon a law 

 of animal ontogeny or individual development of wide ap- 

 plicability and this law, looked at from the standpoint of 

 the full-layered stage, is found to reach in both directions, 

 i.e., backward to the mode of origin of the layers from the 

 single undivided egg-stage of the organism, and forward to 

 the mode of origin of the tissues and organs from the layers. 

 Because of the great measure of uniformity among many 

 groups of animals which pervades the passage of the em- 

 byro from the egg-stage to the full-layered stage, embry- 

 ologists have been able to recognize and so name several 

 stages, the descriptions of which are in many cases very 

 clear and precise. The best defined of these are the morula 

 or cell-cluster stage, the blastula or one-layer stage, and the 

 gastrula or two-layer stage. 



On the other hand, looking from the full-layered stage 

 toward the completed organism, a dominant uniformity in 

 developmental procedure, i.e., a conspicuous law of onto- 

 genesis, is seen in the part contributed by each layer to the 

 completed animal. Since it is this aspect of the matter that 

 particularly concerns us, we must go into a little more 

 detail. As laid down in the standard text-books of embry- 

 ology, three layers are recognized, namely the outermost, 

 called the ectoderm; the middle, called the mesoderm (in 



