598 ZOOLOGY sect, 



A " primitive knot," the nature of which is doubtful, appears on 

 the blastoderm, and an invagination is formed, the cavity of which 

 subsequently opens below into the cavity of the yolk-sac (c/. 

 Reptilia, p. 365) and which becomes the neurenteric canal. Con- 

 temporaneous with this invagination is a primitive streak whicii 

 is separated from the " primitive knot " by a distinct interspace 

 in which the blastoderm is not specially differentiated. 



G-eographical Distribution, — The Monotremes are entirely 

 confined to Australia, Tasmania, and New Guinea. The Mar- 

 supials are most abundantly represented in the Australian 

 region, the greater number of the Australian families and 

 genera being restricted to the Australian continent and to 

 Tasmania, though several genera extend to New Guinea and some 

 of the neighbouring islands. The family Didelphyidse, or Opos- 

 sums, inhabits South America and extends into the southern 

 part of North America ; and a single genus, Gcenolestes, of a 

 family usually stated to be related to the Australian Dipro- 

 todonts, has been comparatively recently found in South America. 



The Edentates are most numerously represented in South and 

 Central America, the true Anteaters, the Sloths, and the Arma- 

 dillos being all inhabitants of that region. But the Scaly Ant- 

 eaters and the Aard-varks (Cape Anteaters) are denizens of the 

 Old World ; the former inhabiting Southern Africa and South- 

 Eastern Asia, the latter being confined to Africa. 



The Cetacea are cosmopolitan in their distribution : the great 

 majority are marine, but some ascend rivers, and a few are exclu- 

 sively fluviatile, inhabiting the rivers of South America and 

 South-Eastern Asia. 



The distribution of the Sirenia is somewhat restricted. The 

 recently extinct Bhytina inhabited Behring's Straits. The 

 Manatee is confined to the Atlantic coasts of South America and 

 of Africa, living chiefly in the larger rivers. The Dugong occurs 

 on the east coast of Africa, in the Red Sea, the Indo-Malayan 

 islands, and the northern coast of Australia. 



The Ungulata occur in all the great regions, with the 

 exception of the New Zealand, Polynesian, and Australian. 

 Oxen are, with the exception of the American Bison, natives 

 of the Palsearctic, Ethiopian, and Oriental regions. Wild Sheep, 

 with the exception of one African and one North American 

 species, are confined to the Nearctic and Oriental regions. 

 Goats are also mainly Nearctic and Oriental. Antelopes are 

 confined to the Old World, and are by far more numerous in 

 the Ethiopian than in other regions. The Prongbucks are 

 Nearctic; the Giraffes exclusively Ethiopian. Deer are widely 

 distributed in the Nearctic, Neotropical, Palsearctic, and Oriental 

 regions, but are absent from the Ethiopian. The Camels are 



