PROGENITORS OF MAN, 285 
SECOND HALF OF THE SERIES OF HUMAN ANCESTORS, 
VERTEBRATE ANIMAL ANCESTORS OF MAN 
(Vertebrata). 
NintH Stace: Skull-less Animals (Acrania). 
The series of human ancestors, which in accordance with 
their whole organisation we have to consider as Vertebrate 
animals, begins with the Skull-less animals, or Acrania, of 
whose nature the still living Lancelet (Amphioxus lanceo- 
latus, Plate XII. B, XIII. B) gives us a faint idea. Since 
this little animal in its earliest embryonal state entirely 
agrees with the Ascidia, and in its further development 
shows itself to be a true Vertebrate animal, it forms a direct 
transition from the Vertebrata to the Invertebrata. Even 
if the human ancestors of the ninth stage in many respects 
differed from the Amphioxus—the last surviving representa- 
tive of the Skull-less animals—yet they must have resembled 
it in its most essential characteristics, in the absence of head, 
skull, and brain. Skull-less animals of such structure—out 
of which animals with skulls developed at a later period— 
lived during the primordial period, and originated out of 
the Himatega of the eighth stage by the formation of the 
metamera, or body segments, as also by the further differen- 
tiation of all organs, especially the more perfect develo»ment 
of the dorsal nerve-marrow and the spinal rod lying below 
it. Probably the separation of the two sexes (gonochorism) 
also began at this stage, whereas all the previously men- 
tioned invertebrate ancestors (apart from the 3—4 first 
