656 Comparative Morphology of Chordates 



orchid testes are sterile. By experimental operation, testes of guinea 

 pigs and rats have been confined to the abdominal cavity. They become 

 sterile. The sterility is attributed to the fact that the abdominal 

 temperature is higher than scrotal temperature. In general, however, 

 it is clear that testes do not require lower temperatures than ovaries. 

 In the many mammals whose testes are permanently abdominal and 

 in birds whose abdominal temperatures are higher than that of any 

 mammal, the testes operate quite successfully. The scrotum is a 

 problematic structure. 



Cloacal Region 



In all amniote embryos the posterior region of the endodermal tube 

 expands to become a cloaca. The intestine opens into the cloaca 

 dorsally. The Wolffian and Miillerian ducts open into it ventrally, 

 and the ventral wall gives rise to the embryonic allantois. Later, the 

 base of the allantoic stalk is more or less involved in the development 

 of the permanent urinary bladder from the floor of the cloaca. 

 Before the embryonic cloaca has acquired an external aperture, its 

 posterior endodermal wall grows backward until it comes to lie against 

 the ectoderm, thus producing a two-layered membrane consisting of 

 the joined endoderm and ectoderm. During the earlier period of 

 development, this cloacal membrane separates the cloacal cavity 

 from external space (Fig. 494A). 



In monotremes the embryonic cloacal membrane is eventually 

 perforated and the cloaca acquires a single wide external aperture. 

 Otherwise it persists in the adult without important modification. The 

 anus opens into it dorsally and its ventral region is urinogenital (Fig. 

 496). These egg-laying mammals therefore have a cloaca essentially 

 like that of reptiles (Fig. 497) and birds. The name "monotreme" 

 refers to the fact that the animal has only one posterior external 

 aperture. 



In all modern mammals other than monotremes, the em- 

 bryonic cloaca, at a stage while the cloacal membrane is still intact, 

 becomes divided into a dorsal and a ventral chamber by a horizontal 

 partition which develops as a backward-growing fold of the anterior 

 cloacal wall (Fig. 494). This partition eventually extends backward 

 to the cloacal membrane, to which it becomes joined. It therefore 

 effects complete separation of the dorsal and ventral cloacal regions, 

 each of which later acquires its independent posterior opening to the 

 exterior. In nearly all placental mammals this horizontal partition 

 later undergoes great expansion dorsoventrally, so that the two cloacal 

 regions are carried apart and the adult dorsal anus is far removed 

 from the ventral urinogenital aperture. The superficial region 



