viii CONTENTS 



CHAPTER XII 

 The Region op the Spinal Cord 



PAGE 



Difference between cranial and spinal regions — Absence of lateral root — Meristic 

 variation — Segmentation of coelorn — Segmental excretory organs — Develop- 

 ment of nepbric organs ; pronepbric, mesonepbric, metanepbric — Excretory 

 organs of Ampbioxus— Solenocytes — Excretory organs of Brancbipus and 

 Peripatus, appendicular and somatic — Comparison of coelom of Peripatus 

 and of vertebrate — Pronepbric organs compared to coxal glands — Origin of 

 vertebrate body-cavity (metaccele) — -Segmental duct — Summary of formation 

 of excretory organs — Origin of somatic trunk-musculature — Atrial cavity 

 of Ampbioxus — Pleural folds — Ventral growtb of pleural folds and somatic 

 musculature — Pleural folds of Cephalaspidse and of Trilobita — Meaning of 

 tbe ductless glands — Alteration in structure of excretory organs which bave 

 lost tbeir duct in vertebrates and in invertebrates — Formation of lympbatic 

 glands — Segmental coxal glands of arthropods and of vertebrates — Origin of 

 adrenals, pituitary body, tbymus, tonsils, thyroid, and other ductless glands 

 — Summary 385 



CHAPTER XIII 

 The Notochord and Alimentary Canal 



Relationship between notocbord and gut — Position of unsegmented tube of 

 notocbord — Origin of notocbord from a median groove — Its function as an 

 accessory digestive tube — Formation of notocbordal tissue in invertebrates 

 from closed portions of tbe digestive tube — Digestive power of tbe skin of 

 Ammoccetes — Formation of new gut in Ammoccetes at transformation — 

 Innervation of the vertebrate gut — The three outflows of efferent nerves 

 belonging to v tbe organic system — The original close contiguity of the 

 respiratory chamber to the cloaca — The elongation of the gut — Conclusion 433 



CHAPTER XIV 



The Principles of Embryology 



The law of recapitulation — Vindication of this law by tbe theory advanced in 

 this book — The germ-layer theory — Its present position — A physiological 

 not a morphological conception — New fundamental law required — Com- 

 position of adult body — Neuro-epitbelial syncytium and free-living cells — 

 Meaning of the blastula — Derivation of the Metazoa from the Protozoa — 

 Importance of the central nervous system for Ontogeny as well as for 

 Phylogeny — Derivation of free-living cells from germ-cells — Meaning of 

 coelom — Formation of neural canal — Gastrula of Ampbioxus and of Lucifer 

 — Summary 455 



