xix. 9 EGGS OF MONOTREMES 553 



coracoid and 'precoracoid'. There is no spine on the scapula. The 

 ventral portion of the pelvic girdle is enlarged by the development of 

 epipubic bones, presumably partly for the support of the marsupial 

 pouch. There is a large and broad humerus in both animals, held 

 horizontally. In Omithorhynchus the fibula is expanded at its upper 

 end like the ulna of other mammals, for the attachment of the large 

 muscles that produce the swimming action. 



The condition of the skeleton is therefore quite sufficient to establish 

 the early divergence of the monotremes from other mammals and we 

 are justified when looking at the soft parts in supposing that many of 

 the characters were possessed by the synapsid reptiles. However, it 

 must always be borne in mind that many of the 'mammalian' charac- 

 ters could conceivably have been produced by parallel evolution, sub- 

 sequent to the divergence of the two lines. 



Perhaps the outstanding non-skeletal feature is the egg-laying 

 habit. The large yolky eggs have a whitish shell and in the spiny ant- 

 eater are transferred by the mother to a special marsupial pouch, 

 which develops at the breeding season. The female platypus makes a 

 nest in a burrow for her two or three eggs and remains with them 

 continuously until after hatching. Monotremes are unique in possess- 

 ing a caruncle on the head as well as the egg-tooth, suggesting that 

 both were present in the ancestor of Amniotes as means of breaking 

 out of the shell. After incubation and hatching the young enter the 

 pouch and are fed by milk. The post-natal care of the young therefore 

 developed before the egg-laying habit was lost. Both genera produce 

 milk from specialized sweat glands on the ventral abdominal wall of 

 the female, but the ducts of these are not united to open on nipples. 



The presence of hair again gives us a valuable clue. Unless this 

 feature has been separately evolved on several lines we may conclude 

 that the Mesozoic mammals and perhaps even the synapsids had made 

 some progress in temperature regulation. The mechanism is still 

 imperfect in monotremes, whose temperature is lower and more 

 variable than that of other mammals. The platypus has a fine short 

 fur of dark brown colour; in the spiny ant-eater the back carries a 

 mixture of spines and hairs, the belly carries hairs alone. 



The rectum and urinogenital system open to a common cloaca, a 

 'reptilian' feature found also in marsupials (Fig. 328). The testes are 

 undescended. The penis of the male is a simple groove in the cloacal 

 floor and is used only for the passage of sperm, the urine entering the 

 cloaca by a special urinary canal. A curious feature found in both 

 monotremes but in no other mammals is a grooved poison spine on 



