PROGENITORS OF MAN. 285 



SECOND HALF OF THE SEMES OF HUMAN ANCESTORS. 

 VERTEBEATE ANIMAL ANCESTORS OF MAN 



( Vertebrata) • 



Ninth Stage : 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 development 

 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 aU the previously men- 

 tioned invertebrate ancestors (apart from the 3 — 4 first 



