INTEGUMENT 



35 



modified integumentary glands which have a certain similarity to 

 sweat glands, and like these, are probably specialised forms of 

 primitive tubular glands. The sebaceous glands arise much later 

 ontogenetically, but it is interesting to note that in the develop- 

 ment of the mammary area, traces of early stages of hairs may be 

 observed : these disappear later, but their sebaceous glands become 

 connected with the mammary ducts. 



Potentially, therefore, mammary organs may be developed in 

 any part of the skin, but as a matter of fact, they are limited to 

 the ventral side in adaptation to the 

 method of suckling the young. ^ ^ z 



Amongst the oviparous Mono- 

 tremes, a marsupial pouch appears in 

 the embryo of Echidna as an infold- 

 ing of the abdominal wall (Fig. 26). 

 This pouch, which serves to shelter 

 the egg and young, becomes tempor- 

 arily enlarged in the breeding season 

 as the offspring increases in size, 

 and has the form of a deep sac ex- 

 tending backwards and provided 

 with closing muscles. On its lateral 

 walls are a pair of depressions, the 

 so-called mammary pockets, which also 

 arise periodically. A bunch of hairs 

 is present in each pocket, the follicles 

 of which open along with the mam- 

 mary ducts on these two glandular 

 areas, which are sharply marked off 

 from the rest of the pouch. The 

 mammary glands themselves, which 

 are almost equally developed in the 



two sexes, consist in Echidna of long, coiled, and much branched 

 tubes, the blind ends of which are swollen. Both they and the 

 mammary pockets are acted upon by a strong integumentary com- 

 pressor muscle, the presence of which is all the more necessary 

 as Monotremes possess no teats. The manner in which the young 

 take in the milk is uncertain : it has been supposed that the milk 

 drops from the two bunches of hairs and is then licked off; or 

 it may be that a temporary teat is formed by suction. 1 



Amongst Marsupials, in which teats are present, the pouch is 

 evidently homologous with that of Echidna, but it reaches a higher 



1 Nothing definite is known as to the manner in which Ornithorhynchus cares 

 for its young. The eggs are laid in burrows in the earth, and it appears that 

 no marsupial pouch is formed at any time ; possibly it may have gradually 

 disappeared as the animal acquired an aquatic habit, The sieve-like apertures 

 of the mammary gland are distributed over two spindle-shaped areas on the- 

 ventral body-wall which are covered by the fur and surrounded by integumentary 

 muscles. 



D 2 



FIG. 27. RUDIMENT OF THE 

 MARSUPIUM IN Diddphys 

 marsupialis, RECONSTRUCTED 

 FROM TRANSVERSE SECTIONS. 

 (After Bresslau. ) 



M, marsupial pockets ; Z, teats 

 or mammary pockets ; f, 

 lateral walls of the marsu- 

 pial pockets which fuse to 

 form the walls of the pouch. 



