36 



COMPARATIVE ANATOMY 



stage of development. A solid ring-like ridge of the epiderm is 

 formed around each teat (or mammary pocket from which the teat 

 arises), and later on the areas thus enclosed sink inwards so as to 

 give rise to a row of hollows or marsupial pockets, which together 

 constitute what has been called the marsupial line (Fig. 27). By 

 a fusion of the adjacent lateral walls of the pockets arise the 

 marsupial folds which form the pouch (Fig. 28). The lowest 

 Marsupials show no trace of a pouch, and their ancestors probably 

 never possessed one. 



Marsupial pockets persist permanently in various degrees of 

 perfection in a number of the Eutheria ( e.g. Manidae, Murida3, 



FIG. 28. POUCH OF Thylacinus, AFTER REMOVAL OF THE SKIN. 

 (After Cunningham.) 



c, compressor mammae (^cremaster of male), passing over which are seen blood- 

 vessels and the genito-cmral nerve ; /, lymphatic glands ; s, sphincter 

 marsupii ; z, teats. 



Cervidse, Carnivora). In Mice they can be recognised until the 

 beginning of lactation, and are then evaginated and thus lengthen 

 the teats. 



The teats may become developed in one of two ways. Either 

 the skin surrounding the mammary pocket (Fig. 29, A) becomes 

 raised up to form a circular rampart, and thus gives rise to a teat 

 perforated by a canal, into the base of which the ducts of the 

 gland open (B) ; or the gland surface itself becomes elevated into 



