PHYSIOLOGY OF SEX AND REPRODUCTION. 233 



irritated surface. Yet the increased circulation may also be viewed 

 as increased by the sitting; in any case, the patience and solicitude of 

 the brooding, and the subsequent diligence in feeding the hatched 

 young, are obviously the expression of genuine parental affection. 



Here, too, one must include the retention of the young in skin- 

 pouches, exhibited by the great majority of marsupial mammals and 

 bv the echidna. In the latter, the pouch is a simple and possibly 

 periodic structure, arising from an insinking of the skin in the mam- 

 mary region of the abdomen. Here the eggs are somehow or other 

 stowed away and the young developed. The milk-glands simply open 

 on the surface of the depression. In most marsupials, the young, 



Fig. 89.— The female Surinam Toad, with young ones on its back. — From Leunis. 



which are born precociously after a very short uterine life, are sheltered 

 in similar, but more developed, pouches of the skin, within which the 

 teats open. 



In oviparous reptiles, the eggs are usually left to hatch of them- 

 selves, aided by the warmth of sun and soil. ' ' The female python 

 disposes herself in coils round her eggs, and incubates them for a 

 prolonged period, during which the temperature has been observed to 

 rise as high as 96 F. within the coils." 



Some exceedingly curious parental adaptations occur among 

 amphibians, which seem to have made numerous experiments on the 

 matter. Thus in the Surinam toad (Pifia), the male spreads the ova 



