4 CLASSIFICATION OF THE MAMMALIA. [CHAP. 



chamber, and there is no distinct vagina. The testes of the 

 male are abdominal in position throughout life, and the vasa 

 deferentia open into the cloaca, not into a distinct urethral 

 passage. The penis, attached to the ventral wall of the 

 cloaca, is perforated by a canal in the greater part of its 

 length, but not at the base, which is open, as in reptiles and 

 those birds which have such an organ, and brought only 

 temporarily in contact with the termination of the vasa 

 deferentia, so as to form a seminal urethra when required, 

 but never transmits the urinary secretion. This condition 

 is a distinct advance on that of the Sauropsida in the 

 direction of the more complete development of these 

 parts in most of the other Mammalia. The ureters do not 

 open into the bladder, but behind it in the dorsal wall of 

 the genito-urinary passage. The mammary glands have 

 no distinct nipple, but pour out their secretion through 

 numerous apertures in the skin ; the whole mammary area 

 is restricted to a slightly cup-shaped invagination of the 

 abdominal skin, forming a mammary marsupium, especially 

 developed in the females during lactation, but hardly recog- 

 nisable in the males. The Monotremata lay eggs and hatch 

 them in the abdominal pouch. The primary egg undergoes 

 meroblastic segmentation as in the Sauropsida ; the early 

 stages of the development of the young are not yet fully 

 known, but they appear never to be nourished by means of 

 an allantoic placenta. 



II. The Didelphia, Marsupial ia, or Metatheria are repre- 

 sented at present by numerous species, presenting great 

 diversities of general appearance, structure, and habits, al- 

 though all united by many essential anatomical and physio- 

 logical characters, which, taken altogether, give them an inter- 

 mediate position between the Prototheria and Eutheria. In 

 the structure of the brain and the presence of epipubic bones 



