URINOGENITAL ORGANS 



359 



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

 bladder. 



Z2K 



FIG. 285. A, RIGHT KIDNEY OF A DEER; B, KIDNEYS (X) AND SUPRARENAL 



BODIES (JN T .A T ) OF THE HUMAN EMBRYO. (Both from the ventral side.) 



Ur, ureters. 



GENERATIVE ORGANS. 



In Amphioxus the gonads are developed in a part of the 

 reduced coelome 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 urinogenital 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 mesorchinm 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 



