WHAT IS AN ORGANISM? 1 73 



tiated tissues from which develop, as growth proceeds, the 

 special organs. There is also formed very early in the de- 

 velopment of most of the higher animals, the Metazoa, an 

 intermediate layer called the Mesoderm. 



These several stages of histogenic development distinguish 

 the Metazoa from the Protozoa, and the distinction might be 

 stated by describing the Protozoan as a cellular animal, the 

 Metazoan, as a tissue-bearing animal. 



The Special Organs Arising from Primitive Tissue Layers. 

 This is not the place to go into further details regarding the 

 mode of development of organisms, but, as illustrative of the 

 degree of specialization of function already outlined in the 

 distinction of the tissues of the gastrula into Ectoderm, Endo- 

 derm and Mesoderm, the following summary of the organs 

 which develope in the Vertebrate from each of the primitive 

 tissue layers is given. 



1. From the Ectoderm arise the epidermis, the nervous 

 system, and the infoldings at each end of the intestinal cavity. 



2. From the Endoderm arise the mesenteron and its ex- 

 tensions, the lung, liver, etc., and the notochord (later, the 

 backbone). 



3. From the Mesoderm arise dermis, muscles, connective 

 tissue, bony skeleton, and probably the reproductive organs. 



The Embryo Stage, characterized by Dependence and Passivity, 

 is not subject to Individual Struggle for Existence. Fifth. The 

 stages of development, enumerated under the preceding 

 section, take place either within the cavity of the parent 

 body or within a food-holding case provided by the parent ; 

 in other words, the organism is not free, building up its 

 growth by its own energies, but it is still attached and de- 

 pendent upon the vital conditions and resources of the parent. 

 It is called a germ, and the embryo stage of development. 



In the development of each metazoal animal there is this 

 dependent stage of development, the embryo stage, of greater 

 or less length, in which the young organism is not an inde- 

 pendent individual, and therefore is not subject to the action 

 of struggle for existence. 



The most important fact to note regarding this stage, is, 

 that it is the stage in which all the differentiation of tissues 



