CONTENTS. 



I. Stomodaeum and medullary Tube p. 1 



Ascidians, p. 2 — Segmented Invertebrates, p. 2 — Anne- 

 lids, p. 2 — Other theories, p. 4 — The old mouth, p. 4 



— Protostomians and Deuterostomians, p. 5 — My theory, 

 p. 6 — Meaning of the medullary tube, p. 7 — Medullary 

 tube and stomodaeum, p. 8 — Neuropore, p. 11 — Gaskell, 

 p. 11 — Conversion of stomodaeum into medullary tube, p. 12 



— Spinal ganglia, p. 14 — Spinal nerves, p. 15 — Muscu- 

 lature and its innervation, p. 17 — Double innervation, p. 18 



— Spinal ganglia in Amphioxus, p. 19 — No stomodaeum 

 in Chordates, p. 20 — Taste in Chordates, p. 20 — Proto- 

 stomia, Deuierostomia, Tritostomia, p. 21. 



II. Origin and Structure of the Head p. 22 



Brain and cerebral ganglia, p. 22 — Praeoral lobe in 

 Annelids, p. 23 — Mesenchyme and coelomesoblast, p. 24 



— Praeoral lobe in Amphioxus, p. 25 — Secondary mesoblast 

 in the prostomium, p. 26 — Praechordal brain in Craniates, 

 p. 27 — Optic organs of Acrania, p. 27 — Attempts to derive 

 the Craniate eyes from them, p. 28 — Eyes of Annelids and 

 Molluscs, p. 29 — Absence of paired eyes in Acrania, p. 31 



— Optic pits in Craniates, p. 31 — Encephalogenetic origin 

 and inversion of Craniate eyes, p. 32 — Homology of Cra- 

 niate and Protostomian eyes, p. 34 — Situation of the animal 

 pole, p. 35 — Hatschek's view on the Craniate brain, p. 36 



— Archencephalon and deuterencephalon, p. 37 — Neuropore 

 of Craniates and of Amphioxus, p. 39 — Cranial flexure, p. 

 39 — Auditory organs in Acrania, p. 39 — Organs of equi- 

 librium of Annelids and Molluscs, p. 39 — Auditory vesicles 

 in Craniates, p. 41 — Praeoral lobe in Craniates, p. 41 — 

 Olfactory organ, p. 43 — Ciliated pits in Annelids and 

 Molluscs, p. 44 — Homology of olfactory pits of Proto- 

 stomians and Craniates, p. 46 — Three main sense organs 

 inherited from Annelids, p. 46 — Brain of Annelids and 

 Craniates, p. 48 — Primary brain axis, p. 50 — Diiferent 

 situation of the neuropore in Urodelans and AnuranSj p. 51 — 



