360 



Vertebrata. 



epididymis. This consists of numerous coiled tubes, and lies 

 close to tlie testis receiving the spermatozoa from it, and carrying 

 them to the segmental duct, which serves as the seminal duct 

 (vas deferens) of the adult. The posterior portion of the 

 embryonic kidney atrophies, and the epididymis loses its original 

 excretory function. It follows, from this description, that the 

 embryonic kidneys of Reptilia, Aves, and Mammalia correspond 

 with the permanent organs of Pisces and Amphibia, whilst those of 

 the adult are, on the other hand, quite new structures. 



A pair of organs lying far anterior in tte body-cavity is distinguished by the 

 term head kidneys (pronephros) * -. each consists of one or more glandular 

 tubes. One end of each tube opens by a funnel into the 

 body-cavity, the other is connected with a duct, the 

 segmental duct, opening into the cloaca. Opposite the 

 funnels a large glomerulus projects from the body-wall. 

 This head kidney appears in the embryo as yet another 

 urinary organ, but before long it gradually disappears,t 

 the duct, however, remains as the ui-eter of Fish and 

 Amphibia, and the Wolffian 'duct of higher Vertebrata. 

 In the latter the pronephros is rarely functional, and 

 is very poorly developed ; in Amphibia and many I'ish 

 on the other hand, it is for some time the functional 

 excretory organ both in the embryo, and sometimes also 

 in the young animal. 



In the Vertebrata there is usually a pair of supra- 

 renal bodies (sometimes separated into several 

 portions), organs whose significance is still unknown. 

 They are mentioned here since they usually lie close to 

 the kidneys, with which, however, they do not come into 

 any further relation. The suprarenal organs are usually 

 yellow or brown in colour, and consist of connective 

 tissue, in which are embedded strings of cells or vesicles. 

 They are very vascular, and well supplied with nei-ves. 



The Vertebrata typically possess two ovaries, 

 although in many Fish they are fused, and one 

 is generally abseut from Birds ; they are firmly 

 attached to the dorsal body-wall, and are covered 

 by a simple epithelium, from which there are 

 inpushings into the subjacent connective tissue even in early 

 developmental stages. These invaginated regions separate from 

 the superficial epithelium as small rounded groups of cells, in 

 which there is a central larger cell, surrounded by a sheath of 

 smaller ones. Such a group is termed a Graafian follicle, 

 the central cell is the young ovum, which gradually increases, and 



Fig. 290. Diagram of a 

 pronephros, a aorta, 

 g — g (left) glomeruli, g 

 (below and to the right) 

 duct of embryonic kid- 

 ney, t funnels. — Orig. 



* In relation with this, the Mdney of Fish and Amphibia, and the embryonic kidney 

 of the higher Vertebrata may be termed the mesonephr<is, and the adult kidney of the 

 latter, the metanephros. 



f In certain Teleostei it persists throughout life as an excretory organ ; in many 

 others it persists in a modified Qondition, biit in the adult bq longer excretes urine, 



