36 GENESIS OF MAN. 



The Protozoa are not a co-ordinate department or type with the 

 Vertebrates, Mollusks, etc. They constitute a sub-kingdom, co- 

 ordinate only with the other sub-kingdom of Mctazoa, under which 

 these types all fall. To class the Protozoa among the Radiates 

 would be equivalent to placing the Cryptogams under the Endo- 

 gens in a botanical system. The reasons for this are purely onto- 

 genetic. The Gastrida possesses the two primary germinative 

 layers, which belong to none of the forms below it. The most 

 thorough embryological research has established beyond a doubt 

 the important fact that all the tissues of the body of every animal 

 that develops beyond that stage, are evolved out of the one or the 

 other of these primary layers. The Protozoa and the Metazoa are 

 therefore separated by the broadest possible line of demarkation, 

 the' former possessing no primary germinative layers, while the 

 latter are either composed of them or developed out of them. - 



The extreme importance of these cellular layers, therefore, be- 

 comes at once apparent, and it is upon the manner in which the 

 different tissues of the body are formed out of this simple building 

 material that the most patient and indefatigable embryologists 

 have been engaged during the past half century. It is found that 

 from the outer layer or exoderm are formed : first, the epidermis 

 and organs arising from it (hair, nails, feathers, scales, etc.) ; sec- 

 ondly, the nervous system and the most important part of the 

 organs of sense ; thirdly, the greater part of the flesh of the body, 

 the muscles ; and fourthly, the skeleton of vertebrates ; in short, 

 all the organs of locomotion and sensation. 



Out of the inner layer, or entoderm, on the other hand, are de- 

 veloped first, the inner lining or epithelium of the entire cavity 

 of the body, together with that of all the glands and organs be- 

 longing to it, lungs, liver, etc. ; secondly, the muscles of the in- 

 ternal vegetative system, including the heart ; and thirdly, the cells 

 of the generative organs. This last, however, is still open to some 

 doubt. 



In consequence of these special functions performed by each of 

 the two primary germinative layers, the outer one has been called 

 the animal germ-layer {animales Keimblatt), and the inner the vegeta- 

 tive germ-layer (vegetatives Keimblatt). The Latin terms exoderma, 

 dermophyllum, lamina dermalis, and lamina serosa, have also been 

 applied to the former, and cntoderma, gastrophyllum, lamina gas- 

 tralis, and lamina mucosa to the latter. 



