GENITAL ORGANS 463 



and anal apertures. The bladder of the Eutheria is thus mainly, 

 though not entirely, of endodermal origin, and the base of the 

 allantoic stalk into which the ureters open becomes included in it. 



GENITAL ORGANS. 



In Amphioxus the gonads are developed in a part of the 

 reduced coelome situated on either side of the pharynx and 

 intestine between the outer body- wall and the atrial cavity. They 

 have a marked segmental arrangement, and each portion sheds its 

 products independently into the atrial cavity, whence they pass 

 out through the atrial pore (cf. p. 352 and Fig. 258). The sexes 

 are separate, and are easily distinguishable from one another when 

 the sexual products are ripe, although the gonads have a similar 

 form in both sexes. The gonads, which are said to act also as 

 excretory organs, are surrounded by blood-spaces which communi- 

 cate with the portal system (Fig. 324). 



In Cyclostomes also, generative ducts are wanting ; the sper- 

 matozoa or ova are shed directly into the body-cavity, and pass 

 through the genital pores (p. 389) into the urinogenital sinus, 

 which in Lampreys is produced into a papilla on the apex of 

 which the urinogenital aperture is situated. The gonad is a long, 

 usually unpaired organ suspended, as in other Vertebrates, to the 

 dorsal wall of the body-cavity by a fold of peritoneum, the 

 mesoarmm or mesorchium. 1 



Myxine is hermaphrodite, and the adult is either predominantly 

 male or female. The posterior part of the gonad represents a 

 testis, and the remainder an ovary, and either the one or the 

 other becomes mature in each individual. 2 



In the true Fishes the gonads are only exceptionally unpaired, 

 and even then this is only a secondary condition, due to the fusion 

 of the two organs or to the reduction of that of one side : as in all 

 other Vertebrates, they are originally paired. There is sometimes 

 a want of symmetry observable between the organ of the right and 

 left side respectively. 



In the greater number of Elasmobranchs the ovaries are 

 paired : but in some cases (e.g. Scyllium, certain Rays) only that 

 of one side becomes developed. The oviducts, as already men- 

 tioned (p. 452) correspond to Miillerian ducts. Their anterior 

 portion has a common opening into the body-cavity, and further 

 back each is provided with an oviducal or shell- gland. The anterior 

 part of the oviduct is always narrower and more delicate than the 

 posterior, which dilates to form a kind of uterus in which, in vivi- 

 parous forms, the embryo undergoes development. Posteriorly, 

 the oviducts open into the cloaca somewhat behind the aperture of 



1 In jVtyxinoids the genital ridge is occasionally paired. 



2 Sterile forms are also said to occur. 



