148 Bulletin American Museum of Natural History. [Vol. XXVII, 



(3) Localization and hypertrophy of the protolacteal glands in the 

 brooding area thereupon followed (Darwin). 



(4) The eggs at first would not be carried along when the mother left 

 the nest; but they might gradually come to adhere to the brooding surface 

 by means : (a) of its depressed character {h) possibly in part by the adhesive 

 quality of the proto-lacteal fluid (o/., the large albuminous element in the 

 milk of Echidna); (c) eventually by means of the raised borders of the 

 brooding area which finally became the marsupium (c/., marsupium in 

 Hippocampus, Dar^^ in) . 



(5) The intimate association between the maternal brooding area and 

 the egg would be a necessary antecedent of the intimate association between 

 the maternal brooding area and the young. 



(6) The mother at first merely settled down over the egg but afterward 

 came to transfer the egg (c/. Echidna) and later the young (c/. Marsupials) 

 to the pouch. 



. (7) In the pro-mammals the eggs were large-yolked (r/. Echidna) and 

 the young were hatched in an advanced condition, as in many reptiles 

 (Darwin). 



(8) These advanced young may have hatched out in the pouch, or 

 after leaving it, may often have been driven back to it to seek for warmth. 

 When the mother moved about they may have clung to the long hairs in the 

 mammary field or buried their heads in this warm bosom. 



(9) At the base of these hairs the protolacteal glands were pouring 

 out their albuminous, oily fluid. This may have served at first to keep the 

 young warm, just as it formerly served to keep the egg warm. 



(10) The milking habit was initiated when the young ones discovered 

 that the material milk secretion tasted good. They would then at first 

 lick it from the base of the hairs (e/. Echidna). 



(11) Licking would gradually give place to sucking and all its complex 

 reflexes, while the nutritive quality of the milk improved. 



(12) The newly born (or hatched) young gradually become virtual 

 external parasites upon the mother. They retain and greatly develop the 

 habit and organs of sucking, which they acquire at a constantly earlier period 

 of development (Darwin). 



(13) The keratin in the egg shell becomes eliminated. 



(14) The maternal teats and marsupium meanwhile are being perfected. 

 In the Marsu})ials, by means of the compressor mammse muscles and other 

 maternal adaptations, the milking efforts of the young are at first assisted 

 and then largely supplanted, so that the young finally become mere larvae, 

 with special larval organs, such as the intranarial prolongation of the epi- 

 glottis. 



