Anatomy. — ''jShori lustory of the head of Vertebrates'' By Dr. H. 

 C. Delsman. (Coinmunicated by Prof. J. Bokkk). 



(Communicated in the meeting of Dec. 23, 1917). 



In preparing a second edition of my theory on the origin of 

 Vertebrates I was led to indulge once more in a number of ques- 

 tions relating to their structure and development. Applying to these 

 the principles of my theory I arrived again at several new points 

 of view. As the completion of the more elaborate article will probably 

 be delayed for some time, I wish to give here a short summary 

 of the views arrived at concerning the structure of the head 

 of Chordates, though I will not add now a complete account of the 

 considerations which induced me to embrace certain conceptions and 

 to reject others. It seems to me that with the records now at hand 

 we have approached considerably nearer to the solution of this problem 

 than one would be inclined to conclude from a more superficial 

 acquaintance with the chaos of divergent and contradictory opinions 

 of older and of more recent investigators. 



The history of the head of Vertebrates is closely related to that 

 of the animal pole of the egg and of the blastula in the animal 

 kingdom. Already in Volvox we find a contrast between two opposed 

 poles of the colonj^ expressed by the stronger development of the 

 red stigmata characteristic of Flagellates at the one pole and of the 

 plasmodesms, serving for the transport of food between the cells, at 

 the other. In the development, too, which begins with an egg cleavage 

 reminding one of the spiral type, the contrast between the two poles 

 becomes evident. The colony swims with the animal pole forward, 

 rotating round the main axis (for literature cf. Janet, 1912). The 

 same holds for the free-swimming blastula — "the animal Volvox" 

 as Huxley (1877, p. 678) called it — of different groups of marine 

 animals, the planula of Coelenterata and other pelagic larvae. The 

 animal half of the blastula as a rule develops into a sensory and 

 nervous centre, the so-called apical plate of larvae like the trocho- 

 phore, arising from the four animal cells of the eight-celled stage 

 (1^^ quartet of micromeres). The animal pole and the prae-oral lobe 

 or prostomium, to which the apical plate gives rise, as a rule 

 continue to indicate the anterior end of the body in free-moving 



70 

 Proceedings Royal Acad. Amsterdam. Vol. XX. 



