MAIN FEATUKES OF MAMMALIAN GEKM-HISTORY. 329 



pression of a general articulation of the body, which is all the more 

 important in consequence of its appearing prior to the articulation of the 

 originally inarticulate axial skeleton. Hence this general articulation may 

 be considered as a primitive vertebral structure, to which the articulation 

 of the axial skeleton is related as a secondary process of the same sort." — 

 Karl Gegenbaub (1870). 



The most important processes, which we have just noticed 

 in the construction of the body from the gerai-layers, are 

 essentially similar in all Vertebrates. In these points Man 

 entirely resembles the other Mammals ; nor do the latter 

 essentially differ from other Vertebrates. It is true that a 

 more exact study of germ-history brings various differences 

 to light, some of which are very striking: among these 

 may be mentioned the formation of a large yelk-sac 

 in most Fishes, in all Reptiles, Birds, and Mammals ; also 

 the formation of the amnion and allantois in the three 

 higher vertebrate classes. But all these remarkable struc- 

 tural conditions, which react on the diversified development 

 of other parts, were only kenogenetically acquired at a later 

 stage, in consequence of Adaptation to the conditions of 

 egg-life ; on the contrary, the most important conditions of 

 the original body-structure, which must be regarded as 

 palingenetic, as transmitted by Heredity from the common 

 parent-form of all Vertebrates, are, on the whole and in the 

 main, everywhere the same. 



As such essential main acts in the germ-history of all 

 Vertebrates, the following must be especially noted : — 1. 

 The formation of a Gastrula (in the most original form in 

 the Amphioxus, in a form which is modified from the 

 latter in all other Vertebrates). 2. The fission of the four 

 primary germ-layers into four secondary germ-layers (often 

 with a three-layered stage intermediate between the two 



