234 THE EVOLUTION OF MAN. 



researches by many observers, embracing the Ontogeny 

 of the most diverse higher animals, have now established 

 the important fact that the germ in a certain stage is 

 composed of four secondary germ-layers. It is most im- 

 portant to notice that this is quite as true of Man as of 

 other Mammals. 



In many cases there is a three-layered stage interme- 

 diate between the two and the four-layered comlition.'^^ 

 But in proportion to the certainty of this conclusion, 

 that there are at first two, and afterwards four layers, it is 

 difficult to understand the way in which these four 

 secondary layers arose from the two primary layers. In 

 this respect the opinions of the many observers who have 

 studied the question are so contradictory that comparison 

 of them fails to enable us to reach the truth. There is, 

 however, no doubt of the one fact, that these four layers 

 result solely from the two original germ-layers, and that 

 they are not partly independent of the latter, as Reichert, 

 His, and other confused observers have asserted. "^^ But the 

 question yet remains undecided whether the two middle 

 layers both originate from one of the two primary layers 

 (from the outer or the inner), or whether one of the two 

 middle layers must be referred to the upj)er, the other to 

 the lower of the primary germ-layers. 



In order to show the importance of this question to 

 the wliole history of evolution, I will now briefly indi- 

 cate the significance of the two middle layers. We must 

 caU these two middle layers the second and the third, 

 numbering the four secondary germ-layers in order from 

 the outer tc the inner. The outer skin, the muscular mass 

 or flesh of the trunk, the muscles, which move the body 



