n6 HISTORY OF THE HUMAN BODY 



extending from axilla to groin, occupying the position of a 

 future row of nipples. By the suppression of this mammary 

 ridge at regular intervals there arises a series of elevations 

 which at first sight appear to be the nipples, but which become 

 secondarily reduced and eventually come to form actual de- 

 pressions. These are evidently the ontogenetic repetitions of 

 the mammary pockets, since from these, after the manner 

 detailed above, the true nipples arise, faithfully repeating the 

 stages shown in Fig. 31. The mammary ridge perhaps repre- 

 sents the wall of the marsupium or pouch, thus suggesting 

 that the placental mammals have been derived from ancestors 

 which possessed a marsupium. The conclusion is not neces- 

 sary, however, that these ancestors were the Didelphia of the 

 present day, but that the common ancestors of both modern 

 types of mammals, marsupial and placental, possessed this 

 organ, and that the Didelphia have retained it as a functional 

 organ, while in the Monodelphia but few traces remain. The 

 occurrence of a marsupium among the monotremes, the only 

 living representatives of the Prototheria, points to the same 

 thing. (See Fig. 8, and the accompanying text.) 



The number and position of the nipples vary much in the 

 different groups of monodelphic mammals, and furnish a 

 series of illustrations of adaptation, both to the habits of life 

 and the number of the young. In that type which appears to 

 be the most primitive, there is a series of nipples arranged in 

 a lateral row upon either side and extending from axilla to 

 groin. In this case, as in pigs, most carnivora, and many 

 rodents, the animal lies upon one side while nursing. By the 

 suppression of the anterior end of this series inguinal mamma 

 are produced, as in ungulates, which nurse their young while 

 standing erect. In the Cetacea the single pair of inguinal 

 nipples lies in the bottom of a pocket, not the primitive one of 

 the Echidna, but one secondarily developed to solve the prob- 

 lem of nursing under water. The lips of this pocket fit tightly 

 about the snout of the young, which can suckle beneath the 

 surface, being at the same time able to breathe through the 

 nostrils, which in these animals have migrated backward from 



