. Quain's elements of anatomy . or extremity is probably one of the peritoneal apertures of that portion ofthe segmental organs. Among the amniota the pronephros or head-kidney is asyet only known in birds by the researches of Sedgwick and Balfour ; but, accord-ing to the views of the latter, it seems probable that the anterior part of Miillersduct is, in reptiles and mammals as well as in birds, homologous with the ductof the pronephros. Although, therefore, the tubular part of the pronephros isnot developed in these animals, there is reason to hold that the type of develop-ment of their urog
. Quain's elements of anatomy . or extremity is probably one of the peritoneal apertures of that portion ofthe segmental organs. Among the amniota the pronephros or head-kidney is asyet only known in birds by the researches of Sedgwick and Balfour ; but, accord-ing to the views of the latter, it seems probable that the anterior part of Miillersduct is, in reptiles and mammals as well as in birds, homologous with the ductof the pronephros. Although, therefore, the tubular part of the pronephros isnot developed in these animals, there is reason to hold that the type of develop-ment of their urogenital passages is, in the main, the same as in birds and thelower vertebrates. The relation of the origin of the productive sexual organs to themesonephros, though not yet fully ascertained, is known to be closerin the male than in the female sex. Thus there is good reason tobelieve that, while in both sexes, as already stated, the productive elementsare derived from the germinal cells of the genital ridge, there are Fig. Fig. )2.—Diagram of the arkangement op the uring-genital organs in an ADULT MALE ELASMOBRANCH (frOTU Balfour). , rudiment of Miillerian duct; Wolffian duet, serving at vcl as vas deferens ;, segmental tubes, two represented with openings into the body cavity ; d, ureter ;t, testis ; nt, canal at the base of the testis ; V. E, vasa eiferentia ; Ic, longitudinal canalof the Wolffian body. differences in the further development of the organs containing thegenerative elements, as in the G-raafian follicles m the one and thespermatic tubes in the other. The External Organs.—With regard to these organs, it is sufficientto remark in this place that the structural elements from wdiich theytake their origin are essentially the same in the two sexes, and thattheir special sexual characters in the male and female depend rather upondifferences in the degree and manner of their development than uponany fundamental disparity in their nature. The nat
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