Vili 
CONTENTS 
CHAPTER XII 
THE REGION OF THE SPINAL CorD 
Difference between cranial and spinal regions—Absence of lateral root—Meristic 
variation—Segmentation of ceelom—Segmental excretory organs—Develop- 
ment of nephric organs; pronephric, mesonephric, metanephric—Excretory 
organs of Amphioxus—Solenocytes—Excretory organs of Branchipus and 
Peripatus, appendicular and somatic—Comparison of ccelom of Peripatus 
and of vertebrate—Pronephric 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 Amphioxus—Pleural folds—Ventral growth of pleural folds and somatic 
musculature—Pleural folds of Cephalaspide and of Trilobita—Meaning of 
the ductless glands—Alteration in structure of excretory organs which have 
lost their duct in vertebrates and in invertebrates—Formation of lymphatic 
glands—Segmental coxal glands of arthropods and of vertebrates—Origin of 
adrenals, pituitary body, ee tonsils, jae and other ductless ae 
—Summary a ‘ : 
CHAPTER XIII 
Tue NorocHorD aND ALIMENTARY CANAL 
Relationship between notochord and gut—Position of unsegmented tube of 
notochord—Origin of notochord from a median groove—Its function as an 
accessory digestive tube—Formation of notochordal tissue in invertebrates 
from closed portions of the digestive tube—Digestive power of the skin of 
Ammoceetes—Formation of new gut in Ammoceetes at transformation— 
Innervation of the vertebrate gut—The three outflows of efferent nerves 
belonging to the organic system—The original close contiguity of the 
respiratory chamber to the cloaca—The elongation of the gut—Conclusion 
CHAPTER XIV 
Tun PrincipLes or EMBRYOLOGY 
The law of recapitulation—Vindication of this law by the 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-epithelial 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 
celom—Formation of neural canal—Gastrula of Amphioxus and of Lucifer 
—Summary . . 7 , ‘ P 
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