THE MONOTREMA, OB CLOACAL ANIMALS. 89 



All the more characteristic is another peculiarity 

 of the Monotrema : the urinary and genital appa- 

 ratus have no separate openings, but one opening 

 in common with the intestinal canal, called the 

 cloaca. This stage of development at which the 

 Monotrema, like the lower Vertebrates, 1 remain all 

 their life, is an embryonal stage in the case of all 

 the other Mammals, and not, as Giebel says, an 

 irregularity or singularity, but a perfectly normal 

 inheritance. In the other Mammals the peculiarity 

 exists normally as a transition form, but after 

 the embryonal life it is a condition that has been 

 overcome. 



That the Monotrema possess actual lacteal 

 glands is a long-established fact. There are a 

 number of separate glands from which the milk 

 issues, not, however, from a teat or nipple, but 

 from flat, perforated patches of skin. These were 

 formerly held to be mucus- or perspiration glands, 

 but are now recognised as an actual proof for the 

 irrefutable supposition that the secretion of milk 

 was acquired only gradually. Those of our readers 

 who may consider this idea of the common skin- 

 glands of the reptile-like animals having in the 

 course of time developed into the important 



1 In most Fishes these arrangements are different. 



