URINOGENITAL ORGANS 



359 



p. 338) the whole allantois takes part in the formation of the 

 bladder. 



W.X 



N.IV. 



rj- 



iri 



Fig. 285.— a, Right Kidney of a Deek ; B, Kidneys {X) axd .Suprarenal 



Bodies (N.X) of the Human Embryo. (Both from the ventral side.) 



U>', ureters. 



Generative Organs. 



In Amphioxus the gonads are developed in a part of the 

 reduced caelome situated on either side of the pharynx and 

 intestine (Fig. 277, a) 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 (compare p. 275 and 

 Fig. 219). 



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. 298) into the uriuogenital sinus, and 

 so to the exterior. The gonad is a long unpaired organ suspended, 

 as in other Vertebrates, to the dorsal wall of the body-cavity by 

 a fold of peritoneum, the mesorchium or mesoarium, as the case 

 may be. 



In 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 



