CHAPTER XII. The Region of the Spinal Cord

THE REGION OF THE SPINAL CORD

Difference between cranial and spinal regions--Absence of lateral root--Meristic variation--Segmentation of coelom--Segmental excretory organs--Development of nephric organs; pronephric, mesonephric, metanephric--Excretory organs of Amphioxus-- Solenocytes--Excretory organs of Branchipus and Peripatus, appendicular and somatic--Comparison of coelom of Peripatus and of vertebrate--Pronephric organs compared to coxal glands--Origin of vertebrate body-cavity (metacoele)--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 Cephalaspidæ 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, thymus, tonsils, thyroid, and other ductless glands--Summary 385