54 THE DOCTRINE OF DESCENT. 



that the Articulata have their navel on their backs. 

 Conversely, it is the characteristic of the evolutionary 

 type of the Vertebrata that the position of the germ 

 corresponds with the dorsal side of the animal. The 

 formation of the dorsal groove, which subsequently 

 closes to form the canal of the spinal cord, as it is 

 gradually enveloped in a sheath growing from below, 

 is followed by the formation of transverse plates, the 

 pre-vertebral plates. The side plates lying outside of 

 these grow towards the ventral side, and finally merge 

 in the navel. The position of the actual vertebral 

 column, consisting of separate vertebrae, is always 

 originally occupied by a cartilaginous band, the noto- 

 chord (chorda dorsalis), and, as from this axis, the germi- 

 nal matter transforms itself into a tube above as well as 

 below, into the spinal marrow with its sheath, and the 

 ventral cavity with the intestinal canal, Von Baer con- 

 sidered this mode of development as bi-symmetrical. 

 The development of the Articulata he regards as simply 

 symmetrical, and the development of the Molluscs 

 he designated as massive. The justification of this is 

 that the elongation produced by segmentation and the 

 repetition of similar parts and sections of the body 

 implicit in segmentation generally, the metameric for- 

 mation, as it is termed by Haeckel, is totally foreign to 

 the Molluscs. 



We must now again repeat, that somewhat extensive 

 observations of the evolutionary forms of different ani- 

 mals lead at once to the belief that the embryos and 

 evolutionary phases of higher animals are transiently 

 more closely related to the complete and definitive con- 

 ditions of the lower animal-forms, at least of the same 



