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	I. Primitively the elongated testis communicated with the Wolfﬁan duct by way of ((t) a series of vasa efierentia distributed along its length, and (b) the tubules of tlie opisthoncphros.		I. Primitively the elongated testis communicated with the Wolfﬁan duct by way of ((t) a series of vasa efierentia distributed along its length, and (b) the tubules of tlie opisthoncphros.

	~~I l~~. The posterior portion of the testis became sterile and functioned merely as a reservoir and duct.		II. The posterior portion of the testis became sterile and functioned merely as a reservoir and duct.

	III. The vasa. etferentia became reduced to those connected with this sterile region and ﬁnally to the hindermost one of these.		III. The vasa. etferentia became reduced to those connected with this sterile region and ﬁnally to the hindermost one of these.
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	==Suprarenal Organs==		==Suprarenal Organs==
	The organ familiar to students of the Amniota and especially of the Mammalia under the name Suprarenal or Adrenal is now generally recognized as being not a single organ but an organic complex formed by the union of two originally separate elements-—the medullary substance and the cortical substance. These two elements arise quite independently i11 ontogeny, the medullary substance being derived from the sympathetic ganglia, while the cortical substance arises in the form of a number of thickenings of coelomic epithelium on the roof of the splanchnocoele between the two kidneys. That this independence in early stages of ontogeny is a repetition of a condition which occurred during phylogeny is indicated by the fact that in Fishes the two elements are still independent. The names medullary and cortical substance, reierring as they do to a topographical relation which occurs only in mammals, are obviously unsuitable from the point of view of comparative morphology and it is becoming customary to apply other more characteristic names. The medullary substance in mammals and what corresponds with it in other Vertebrates has a very characteristic ~~Iv THE SUPRARENAL ORGAN S 283~~		The organ familiar to students of the Amniota and especially of the Mammalia under the name Suprarenal or Adrenal is now generally recognized as being not a single organ but an organic complex formed by the union of two originally separate elements-—the medullary substance and the cortical substance. These two elements arise quite independently i11 ontogeny, the medullary substance being derived from the sympathetic ganglia, while the cortical substance arises in the form of a number of thickenings of coelomic epithelium on the roof of the splanchnocoele between the two kidneys. That this independence in early stages of ontogeny is a repetition of a condition which occurred during phylogeny is indicated by the fact that in Fishes the two elements are still independent. The names medullary and cortical substance, reierring as they do to a topographical relation which occurs only in mammals, are obviously unsuitable from the point of view of comparative morphology and it is becoming customary to apply other more characteristic names. The medullary substance in mammals and what corresponds with it in other Vertebrates has a very characteristic chemical or physical reaction, in that it takes on a deep yellow or brown colour when treated with salts of chromic acid. Hence it is convenient, and usual, to apply to it a name expressive of this reaction-— such as Ghromophile (Stilling), Chromaﬁine (Kohn) or Phaeochrome Poll).

	chemical or physical reaction, in that it takes on a deep yellow or brown colour when treated with salts of chromic acid. Hence it is convenient, and usual, to apply to it a name expressive of this reaction-— such as Ghromophile (Stilling), Chromaﬁine (Kohn) or Phaeochrome Poll).

	( The cortical tissue has also characteristic features——-in particular the fact that its cytoplasm contains numerous granules of lipoid or fat-like substance, soluble in Ether, Xylol, etc., staining deeply with various Aniline stains, and giving the characteristic black with Osmic Acid. For masses of this tissue the name Interrenal organ may be used (Balfour) which although a topographical term like cortical substance has the advantage of being correct for vertebrates in general during at least the early stages of their development.		( The cortical tissue has also characteristic features——-in particular the fact that its cytoplasm contains numerous granules of lipoid or fat-like substance, soluble in Ether, Xylol, etc., staining deeply with various Aniline stains, and giving the characteristic black with Osmic Acid. For masses of this tissue the name Interrenal organ may be used (Balfour) which although a topographical term like cortical substance has the advantage of being correct for vertebrates in general during at least the early stages of their development.
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	Of the more primitive groups of gnathostomatous Vertebrates only the Elasmobranchs and the Amphibians have been studied carefully in regard to the development of these organs and we shall consequently use them as illustrating the general mode of development which, with variations in detail, holds throughout the groups dealt with in this volume.		Of the more primitive groups of gnathostomatous Vertebrates only the Elasmobranchs and the Amphibians have been studied carefully in regard to the development of these organs and we shall consequently use them as illustrating the general mode of development which, with variations in detail, holds throughout the groups dealt with in this volume.

	ELASMOBRANe111I~~.1——The~~ Interrenal organs are here interrenal in position through life, forming either one (Sharks) or a pair (Skates and Rays) of elongated bodies lying in the region of the mesial plane and extending for some distance opposite the hinder part of the opisthonephros.		ELASMOBRANe111I

			The Interrenal organs are here interrenal in position through life, forming either one (Sharks) or a pair (Skates and Rays) of elongated bodies lying in the region of the mesial plane and extending for some distance opposite the hinder part of the opisthonephros.

	In S’c;2/llvlmn (Poll) the interrenal makes its first appearance (7 mm. embryo) in the form of a number of irregularly distributed thickenings of the splanchnic mesoderm in the region of the root of the mesentery, just ventral to the dorsal aorta. The possibility of metameric arrangement in the very earliest stages does not seem to be absolutely excluded but there is no evidence of this so far. The rudiments are most numerous in the genital region but they occur as far forwards as the hind end of the pronephros and back as far as the cloaca. The rudiments of the two sides, projecting towards the median plane, meet and become continuous, and as anteroposterior fusion also comes about, the rudiment takes the form (10 mm. embryo) of a cellular rod lying beneath the dorsal aorta and above the mesenterie root, and for a time still continuous with the splanchnic mesoderm which gave it origin. For a time there is close apposition, amounting to apparent continuity of tissue, between this red and the opisthonephric nephrotomes lying on either side of it, but it is doubtful whether any special morphological signiﬁcance is to be attached to this. In embryos of 16-28 mm. in length the interrenal organ gradually becomes separated in a tailward direction both from the coelomic epithelium and the ncphrotomes, and assumes its deﬁnitive form.		In S’c;2/llvlmn (Poll) the interrenal makes its first appearance (7 mm. embryo) in the form of a number of irregularly distributed thickenings of the splanchnic mesoderm in the region of the root of the mesentery, just ventral to the dorsal aorta. The possibility of metameric arrangement in the very earliest stages does not seem to be absolutely excluded but there is no evidence of this so far. The rudiments are most numerous in the genital region but they occur as far forwards as the hind end of the pronephros and back as far as the cloaca. The rudiments of the two sides, projecting towards the median plane, meet and become continuous, and as anteroposterior fusion also comes about, the rudiment takes the form (10 mm. embryo) of a cellular rod lying beneath the dorsal aorta and above the mesenterie root, and for a time still continuous with the splanchnic mesoderm which gave it origin. For a time there is close apposition, amounting to apparent continuity of tissue, between this red and the opisthonephric nephrotomes lying on either side of it, but it is doubtful whether any special morphological signiﬁcance is to be attached to this. In embryos of 16-28 mm. in length the interrenal organ gradually becomes separated in a tailward direction both from the coelomic epithelium and the ncphrotomes, and assumes its deﬁnitive form.
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	1 The best general account of the development of the Suprarenal organs is that by Poll (1905).		* 1 The best general account of the development of the Suprarenal organs is that by Poll (1905).

	Only the tailward part of the series of original rudiments completes its development in the way described. The whole series extends through about 25 segments but of these only about the posterior half take part in the formation of the interrenal rod: the anterior ones either atrophy completely or develop into small accessory interrcnals.		Only the tailward part of the series of original rudiments completes its development in the way described. The whole series extends through about 25 segments but of these only about the posterior half take part in the formation of the interrenal rod: the anterior ones either atrophy completely or develop into small accessory interrcnals.
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	Zarnik. Zool. Jahrb. (Ana.t.), xxi, 1904.		Zarnik. Zool. Jahrb. (Ana.t.), xxi, 1904.


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CHAPTER iv THE COELOMIC ORGANS

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			=Chapter IV The Coelomic Organs=

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