4C4 



ZOOLOGY 



SECT. 



The mammary glands, by the secretion of which the young are 

 nourished, are specially developed cutaneous glands. 



The mammary glands of the Prototheria differ from those of 

 other Mammals in the absence of teats. They consist of two 

 groups of very large tubular follicles, the ducts of which open on 

 the ventral surface. In Echidna (Fig. 1040) the two areas on 

 which the ducts open become depressed towards the breeding 

 season to give rise to a pair of pouches the mammary pouches. 

 At a later period the part of the body-wall on which these 

 mammary pouches are situated becomes modified to form a 

 marsupium or pouch in which the mammary areas are contained. 



In this, which is of a 



A temporary nature and 



disappears after its func- 

 tion has been performed, 

 the egg is deposited 

 l 



when laid, and in this 

 the young animal, after 

 it has escaped from the 

 egg, is protected and 

 nourished. In Ornitho- 

 rhynchus mammary 

 pouches are indi- 

 cated only by extremely 

 shallow depressions, and 

 no marsupium is de- 

 veloped. 



In the higher Mam- 

 mals, when the mam- 

 mary glands are first 

 developed (Fig, 1041), a 

 depression (mammary 

 pouch} is formed, from 

 the floor of which 

 branching cylindrical 



strands of epidermis grow inwards to give rise to the glands. 

 In some cases (Marsupials, Primates) the mammary pouch dis- 

 appears, and the area on which the glands open is raised up into a 

 conical prominence the teat. In the rest the edges of the mammary 

 pouch grow upwards to form a prominence the false teat (O) 

 with a central canal, into which, at the base, the ducts of the glands 

 open. The number and situation of the true or false teats varies in 

 the different groups, and has been noticed in the synopsis of the 

 characters of the orders and sub-orders (pp. 450 to 460). 



The two genera of the Prototheria, Ornithorhynchus and Echidna, 

 differ somewhat widely from one another in general appearance. 



FIG. 1041. Diagram of the development of the 

 (vertical section). A, indifferent stage, glandu 

 area flat ; B, elevation of the glandular area with the 

 nipple ; C, elevation of the periphery of the glandular 

 area into the false teat. , periphery of the glandular 

 area ; b, glandular area ; gl. glands. (From Gegenbaur.) 



