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 vertebra, 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 
