36 



THE ANCESTRY OF VERTEBRATES 



In Amphioxus, on the contrary, we may expect the animal 

 pole at the same place as in Annelids, i. e. on the anterior 

 surface of the prostomium, a good distance in front of the 

 neuropore. This also is confirmed by the facts. In Amphioxus, 

 it is true, we cannot trace the fate of the animal pole with 

 such certainty as in worms and molluscs, the cleavage 

 being of the indeterminate type which soon renders it 

 impossible to distinguish the individual cells from each 

 other and to determinate from their arrangement the place 

 of the animal pole. The polar bodies, however, often 

 remain attached to the developing egg for a considerable 

 time, up to the gastrula-stage. One of these eggs, as obser- 

 ved by Cerfontaine (1906), is reproduced here in fig. 12. 



If we compare this 



stage with that of fig. 5, 

 it will be evident at 

 once that, if it might 

 be possible to observe 

 the polar bodies in the 

 latter stage, they would 

 be found onthesurface 

 of the prostomium at a 

 considerable distance 

 in front of the neuro- 

 pore. Thus for the 

 third time we reach 

 the same conclusion, 

 viz : that the fore-brain 

 with reference to the 



Fig. 12 Gastrula of Amphioxus, 

 with polar body, 

 after Cerfontaine, 1906 



of Craniates is a new acquisition 

 Acrania. 



Hatschek's view on the Craniate brain. — A similar 

 assumption, combined with the view that the infundibulum 

 represents the primitive fore-end of the brain, comparable 

 to the neuropore of Amphioxus, has, anticipated already by^ 

 V. Baer's investigations, found several adherents and 

 has recently been defended by Hatschek (1909. p. 497). 

 The praechordal part of the brain is imagined by them 

 to be a secondary outgrowth from the epichordal part as 

 found in Amphioxus (cif. the quotation from WiEDERSHEIM 

 anon). According to my theory, however, there is no question 

 of a forward outgrowth of the original epichordal brain of 

 Amphioxus but of a forward extension and an incorporation 

 of ectoderm in front of it. 



