MAMMARY GLANDS IN MONOTREMATA. 767 



it begins to be inverted, loses thickness, and at the fundus of 

 the pouch, ib. c, is only half as thick as where it overspreads the 

 abdomen. 



I have not hitherto met with any trace or beginning of such 

 abdominal pouches in the various Ornithorhynchi in which I have 

 had occasion to note different phases of the development of the 

 ovaria and mammary glands. A warm-blooded air-breather, com- 

 pelled to seek its food in water, could not safely carry the progeny 



603 



Mammary gland, pouch, and young. Echidna Hystrix. cccxxn". 



it had brought forth in a pocket beneath its body during such 

 quest : all observers have noted the nest-making instinct of the 

 Platypus, and in such temporary and extraneous structures only 

 have the young been hitherto found. 



The question remains, whether the marsupial pouches of the 

 Echidna increase with the growth of the young ? It is certain 

 that they only commence with the growth or enlargement of the 

 mammary glands preliminary to birth. In the young specimen of 

 female Echidna in which the glands were first discovered J their 

 ducts opened upon a plane surface of the abdominal integument. 

 In a nearly full-grown unimpregnated female there was also a 

 total absence of inflected folds of the integument where the 

 mammary ducts terminate. Some movement, perhaps, of these 

 ducts in connection with the enlargement of the mammary lobes, 

 under the stimulus of preparation for a coming offspring, may, 

 with associated growth of the abdominal integument surrounding 

 the areola, be amongst the physical causes of the first formation 

 of the pouch. 



The young Echidna, ib. e, resembles the new-born Kangaroo 

 in the proportions of the limbs to the body, in the inferior size 

 and development of the digits of the hinder pair, and in the fee- 

 ble indication of eyes or eyelids. But the mouth is proportionally 



1 cccxxin". p. 179. 



