THE VASCULAR SYSTEM AND VASCULAR GLANDS, pp. 460-478. _The heart._--Its development. Comparison with other types. Meaning of double formation of heart. _The general circulation._ The venous system. The primitive condition of. Comparison of, with Amphioxus and Annelids. The cardinal veins. Relations of caudal vein. _The circulation of the yolk-sack._--Previous observations. Various stages. Difference of type in amniotic Vertebrates. _The vascular glands._--Suprarenal and interrenal bodies. Previous investigations. _The suprarenal bodies._--Their structure in the adult. Their development from the sympathetic ganglia. _The interrenal body._--Its structure in the adult. Its independence of suprarenal bodies. Its development. CHAPTER XII. THE ORGANS OF EXCRETION, pp. 479-520. Previous investigations. _Excretory organs and genital ducts in adult._ _In male._--Kidney and Wolffian body. Wolffian duct. Ureters. Cloaca. Seminal bladders. Rudimentary oviduct. _In female._--Wolffian duct. Ureters. Cloaca.--Segmental openings. Glandular tubuli of kidney. Malpighian bodies. Accessory Malpighian bodies. Relations of to segmental tubes. Vasa efferentia. Comparison of Scyllium with other Elasmobranchii. _Development of segmental tubes._ Their junction with segmental duct. Their division into four segments. Formation of Malpighian bodies. Connection between successive segments. Morphological interest of. _Development of Müllerian and Wolffian ducts._ _In female_--General account. Formation of oviduct as nearly solid cord. Hymen. _In male_--Rudimentary Müllerian duct.--Comparison of development of Müllerian duct in Birds and Elasmobranchii. Own researches. Urinal cloaca. _Formation of Wolffian body and kidney proper._--General account. Details of formation of ureters. _Vasa efferentia._--Views of Semper and Spengel. Difficulties of Semper's views. Unsatisfactory result of own researches. General homologies. _Résumé._ Postscript. CHAPTER I. THE RIPE OVARIAN OVUM. The ripe ovum is nearly spherical, and, after the removal of its capsule, is found to be unprovided with any form of protecting membrane. My investigations on the histology of the ripe ovarian ovum have been made with the ova of the Gray Skate (_Raja batis_) only, and owing to a deficiency of material are somewhat imperfect. The bulk of the ovum is composed of yolk spherules, imbedded in a protoplasmic matrix. Dr Alexander Schultz[70], who has studied with great care the constitution of the yolk, finds, near the centre of the ovum, a kernel of small yolk spherules, which is succeeded by a zone of spherules which gradually increase in size as they approach the surface. But, near the surface, he finds a layer in which they again diminish in size and exhibit numerous transitional forms on the way to molecular yolk granules. These Dr Schultz regards as in a retrogressive condition.